What I want to know is, how many letters were used when they made the scene of hundreds of letters flying from the chimney. Did they use one? Did they use hundreds?
According to Harry Potter - The Unofficial Guide to the Collectibles of Our Favorite Wizard By Eric Bradley:
Invitations to Hogwarts have themselves become instant collector's items. Who can forget the fan favorite moment from Harry Potter and the Sorcerer's Stone film when the owl post inundates the Dursley household with millions of acceptance letters informing Harry of his acceptance into Hogwarts? The scene required actual envelopes, as well as special effects.
Two styles of envelopes were made for production with either a rear wax seal or rear printed seal on a 5-1/2 x 7-1/5-inch envelope. Research from production assistants relating to this prop shows only 1,000 letters were made and re-used and reset for this scene. The values between each can be quite stunning.