Dumbledore uses his Put-Outer to extinguish the 12 street lamps on Privet Drive.
Dumbledore's watch has 12 hands.
The milkman delivers two dozen letter-filled eggs to the Dursleys.
On Friday, 12 letters arrive for Harry.
Hagrid is described as being at least twice the height of a normal man. If a normal man is around 6 feet tall, that would make Hagrid approximately 12 feet tall.
Platform 9 3/4 - 9 is 3/4 of 12
Hogwarts school song has 12 lines.
The troll Quirrell let in on Halloween was 12 feet tall.
During the flying lesson, Neville originally rises to 12 feet.
The Wizards Duel with Draco was supposed to happen at midnight.
At the first Quidditch match, Neville tells Draco he's worth 12 of him.
There are 12 Christmas trees in the Great Hall.
Dumbledore is famous for discovering the 12 uses of Dragon's blood.
At exactly 12:00 AM Harry turns 11, and Hagrid bangs on the door.
Shortly before the trio leaves to go through the trapdoor, Hermione says that she won't get kicked out because she got 112 percent on Flitwick's exam.
Harry hears Quirrell sobbing and giving in to someone in a classrom. After Quirrell leaves the room, Harry says he would bet 12 Sorcerer's Stones that the other person in the room was Snape.
Gryffindor was "in fourth place, with three hundred and twelve points".
JKR said in an online chat that her favorite chapter out of all the books was Chapter 12, Book 1 (The Mirror of Erised)
Chamber of Secrets
Harry turns 12 years old.
After Dobby drops the pudding on the kitchen floor, Harry receives an owl from the Ministry saying that a Hover Charm was used at 12 minutes past nine.
Ron tells Harry that he wrote to him about twelve times over the summer to come and stay with him at the Burrow.
When Draco is complaining about Harry, Lucius cuts him off and says he's heard this a dozen times already.
There are 12 school governors that sign an Order of Suspension for Dumbledore.
Lockhart says there are 12 dwarf cupids roaming around the school delivering Valentines.
Lockhart tells Harry, '... when I was twelve, I was just as much of a nobody as you are now.'
Lockhart says that in Chapter 12 of his book, Wandering with Werewolves, he divulges that his ideal birthday gift is to have peace among all creatures.
We learn that Bill and Percy both got 12 O.W.L.s.
Tom Riddle tells Harry that his mother bought him twelve years of borrowed time.
Prisoner of Azkaban
Aunt Marge has 12 bulldogs.
Sirius escaped from Azkaban after being imprisoned for 12 years.
To get his mind off of Aunt Marge's insults, Harry tries to remember page 12 of his book: "A Charm to Cure Reluctant Reverses."
There were no seats, but half a dozen brass bedsteads on the bottom deck of the Knight Bus.
Harry's Nimbus 2000 was smashed into a dozen pieces.
Fudge says Black murdered 12 Muggles and 1 wizard with a single curse. We should note here that Pettigrew actually DID murder 12 Muggles in total. Thanks to Joel for pointing that out!
At The Leaky Cauldron, Ron and Percy stay in room #12.
Hermione's bag split as it was crammed with at least a dozen large and heavy books.
Buckbeak's wingspan is said to be 12 meters long.
Snape, substituting for Lupin's Dark Arts class, tells the students to turn to page 394 - if we play with those numbers, we get: 3 + 9 = 12, 3 x 4 = 12, 9 is 3/4 of 12. (also see 'References to the Number 7' regarding this number)
Ron tells everyone when he woke, Sirius was standing over him with a 12 inch knife.
At the Christmas feast, Trelawney counts 12 people at the table and refuses to sit down because she would be thirteen.
McGonagall isn't happy when Harry asks about the status of his broom for the 12th time.
Sirius tells Lupin to hurry up with the explanation since he's been waiting 12 years to get back at Pettigrew.
Trelawney's prophecy states "The Dark Lord lies alone and friendless, abandoned by his followers. His servant has been chained these twelve years."
Goblet of Fire
Frank Bryce estimates Nagini to be 12 feet long.
While Percy is lamenting about Rita Skeeter's complaints that they should be stamping out vampires instead of quibbling about cauldron thickness, he says: "As if it wasn't specifically stated in chapter 12 of the Guidelines for the Treatment of Non-Wizard Part-Humans--"
"By 12 o'clock the next day, Harry's school trunk was packed with his school things and all his most prized possessions."
The Basilisk fang from Harry's second year is described as being 12 inches long.
At the Quidditch World Cup, the Bulgarians insisted on having 12 more seats in the Top Box.
Crouch's grandfather used to have a magic carpet that could seat 12.
The letter Harry sends Sirius after the first task is so heavy that when Harry tosses Pigwidgeon out the window with it, he plummets 12 feet before recovering.
The Weasley's grandfather clock shows where each family member might be (such as "work," "traveling," etc.). In the position where the number twelve would be, it states "mortal peril."
As Harry is about to enter the common room after being picked as School Champion, a dozen hands pull him inside.
In the Great Hall at the Yule Ball, instead of the usual four house tables there are about a hundred smaller tables, each seating about a dozen people.
"Harry thought of Sirius, and the tight, tense knot in his chest seemed to ease slightly. He would be speaking to him in just over 12 hours."
Hagrid tells Harry to meet him at midnight, and to bring that cloak with him.
Sirius tells Harry he would need half a dozen wizards at a time to overcome a dragon.
Harry, Ron and Hermione bring Sirius a dozen pieces of chicken.
While Harry and Ron are making up false predictions for Divination, one of their prediction is that Harry will be stabbed in the back by someone he thought was a friend because Venus is in the 12th house.
We learn that Dumbledore has been Hagrid's guardian since he was 12 years old when his dad died.
Mundungus Fletcher put in a claim for a 12-bedroom tent with en-suite Jacuzzi.
Cedric's wand is 12 inches long.
Professor Binns drones on about the Goblin Rebellions of 1612.
12 Durmstrang students arrive for the Triwizard Tournament.
During the final task, the spider drops Harry 12 feet.
Before Harry leaves to get Dumbledore, Crouch tells him that his son Crouch Jr. got 12 O.W.L.s.
Order of the Phoenix
On the American cover there are 12 candles in the foreground.
The headquarters of the Order of the Phoenix is located at 12 Grimmauld Place.
Harry had barely run a dozen steps when he reached Dudley and the dementor.
Harry thinks it's "galling" to be told not to be rash by someone who had spent 12 years in Azkaban.
Harry's disciplinary hearing with the Ministry is on the 12th of August.
Sirius explains to Harry that Voldemort is not going to go up against the Ministry with a dozen Death Eaters.
While Bill is asking Harry if he's alright, he's gathering up 12 scrolls at once.
Roughly half a dozen of the Wizengamot vote in favor of conviction.
In the OOTP, Luna Lovegood is looking at page 57 in the Quibbler; add 5 + 7 to get 12
One of Snape's homework assignments is 12 inches of parchment on the properties of moonstone and its uses in potion-making.
When Hermione is talking to Harry and Ron in the Great Hall, she glances around and sees a half a dozen empty seats around them.
During Harry's detention with Umbridge, he glances out the window to the Quidditch Pitch and sees half a dozen black figures.
At midnight, Harry is in the common room when he starts drifting off and almost gets to the door before he's awakened by Dobby.
Harry sees Dobby picking up a half a dozen hats that Hermione left.
Before Harry first enters Dumbledore's office after seeing the snake bite Mr. Weasley, Harry thinks it sounds like Dumbledore is entertaining at least a dozen people.
When the Weasley kids are at Grimmauld Place worrying about their dad, Sirius summons a half a dozen butterbeers.
"He was exhausted, he was confused beyond measure, he had experienced terror, relief, and then terror again in the last 12 hours, and still Dumbledore did not want to talk to him."
Kingsley asks Mr. Weasley for as much information as possible on flying Muggle vehicle sightings in the past 12 months.
When they meet Lockhart in St. Mungo's, he says: "Well, how about those autographs then? Shall we make it a round dozen?"
Harry is grumpy about having to go back to Hogwarts and place himself under the tyranny of Dolores Umbridge, who had no doubt managed to force through another dozen decrees in their absence.
Harry makes a mental note how much more difficult it is to extend his arm 12 inches and touch Cho's hand than to snatch a speeding Snitch from midair.
Hermione tells Harry to meet her at 12 o'clock in The Three Broomsticks during Harry's date with Cho.
After the article in The Quibbler is published, Trelawney breaks out in tears during one of her classes and exclaims that Harry would not suffer an early death after all, but would live to a ripe old age, become Minister of Magic, and have 12 children.
When James disarms Snape, his wand flies 12 feet in the air.
Harry says he wasn't the sole witness to Voldemort's rebirth, and that there were a dozen-odd Death Eaters there too.
There are a dozen doors in the circular room in the Department of Mysteries.
When the trio are spread out in the shade of the beech tree near the lake, Ron talks them through his first save of the match for what felt like the dozenth time.
In the Care of Magical Creatures O.W.L., the fifth-years have to correctly identify a knarl hidden among a dozen hedgehogs.
Ron reminds Harry that Hermione got 112% on her Charms exam last year.
Ravenclaw sixth-year Eddie Carmichael offers Ron and Harry a whole pint of Baruffio's Brain Elixir for 12 galleons.
Harry pulls Ron's arm over his shoulder, just as he had done with Dudley months ago, and thinks they have a 1-in-12 chance of getting it right this time.
There are a dozen death eaters in the Department of Mysteries.
After Fudge sees Voldemort Disapparate in the Ministry Atrium, Dumbledore tells him it's time he listened to sense, and he's been chasing the wrong man for 12 months.
Dumbledore pulls out his watch with 12 hands and tells Fudge he may have a half-an-hour of his time.
Fred and George's joke shop is at 93 Diagon Alley. 9 + 3 is 12.
As Harry waited for Dumbledore, the minute hand on the alarm clock reached the number twelve and the street-lamp outside went dark.
The bottle of Felix Felicis that Slughorn gives Harry is enough for twelve hours of good luck.
Ginny shouted at Ron that he had about as much experience with snogging as a twelve-year-old.
Hagrid delivered the usual twelve Christmas trees to the Great Hall.
Apparition lessons consist of a twelve-week course. Cost: 12 Galleons.
Harry wishes he hadn't used the Sectumsempra spell on Malfoy ... "and not just because I've got about a dozen detentions."
Snape assigns Harry's detention as clearing out old files beginning with box "one thousand and twelve."
Harry's stomach started rumbling during his detention at half past twelve.
Dumbledore drinks twelve goblets of green potion to empty the basin in the cave.
A dozen giant winged palominos pulled the powder-blue carriage of Beauxbatons to the funeral.
So far, we know of twelve course options offered at Hogwarts: Ancient Runes, Arithmancy,
Astronomy, Care of Magical Creatures, Charms, Defense Against the Dark Arts,
Divination, Herbology, History of Magic, Muggle Studies, Potions, Transfiguration.
(There is a 13th if you include Flying lessons for first years.)
NOTE: In HBP, Hermione receives 11 O.W.L.s AFTER dropping Divination and Muggle Studies, so that would indicate a total of 13 courses. It's either a mistake (gasp!) or there's another course we haven't heard of yet. Thanks to Rory!
Contributing to this section: Ashley, Kelly, Lindsay, Whitney, Joel, Jordon, Ruepert, Corry, Samuel, Kyle, Shaharazad, Becky, Matt, Elizabeth, Erich, Nick