Passage: On November 17, 2004, a teaser DVD named "Air prelude" was produced containing interviews with the anime's cast, clean opening and ending theme video sequences, and promotional footage of the anime itself; it was a limited edition DVD, with only 20,000 copies produced. The anime television series is produced by Kyoto Animation, directed by Tatsuya Ishihara, written by Fumihiko Shimo, and features character design by Tomoe Aratani who based the designs on Itaru Hinoue's original concept. Thirteen episodes were produced by Kyoto Animation: 12 regular episodes, and a final recap episode which summarizes Misuzu's story arc. The anime also follows the game by splitting the series into three parts; Dream (episodes one through seven), Summer (episodes eight and nine), and Air (episodes 10 through 12), with the recap episode (episode 13) following. The episodes aired between January 6 and March 31, 2005, on the BS-i Japanese television network. The theme songs from the Air visual novel are used for the anime's opening theme, ending theme and soundtrack. After the conclusion of the anime series, a mini-series which added to the Summer arc of the story called Air in Summer aired on August 28 and September 4, 2005, a week later on BS-i. Air in Summer consisted of two episodes and was produced by the same staff as the anime series.A DVD released on March 31, 2005, called "Air Memories" contained promotional commercials for the series, staff commentaries, and clean ending sequences from the 12th and 13th episodes, lasting 92 minutes. The episodes were released to Region 2 DVD between April 6 and September 7, 2005, by Pony Canyon in limited and regular editions containing two episodes per volume. The DVD for Air in Summer was later released on October 5, 2005, in Japan. Additionally, Air became one of the first anime series to be released in Blu-ray Disc format on December 22, 2006. A new version of the Blu-ray Disc box set was released on November 28, 2008, in Japan.

Question: How many copies of the teaser DVD were produced?
20,000