57 (number)

57 was a number.

In an attempt to pass Jackie Tyler off as her daughter Rose Tyler to Yvonne Hartman, the Tenth Doctor claimed that she had aged 57 years after looking into the Time Vortex. Jackie, however, insisted that her age was 40. (TV: Doomsday)

When the Twelfth Doctor asked Clara Oswald to pick a two-digit number, she chose 57. He then asked the crew of the crew to look at the first word of page 57 of the book they were holding, as part of the Helman-Ziegler test. (TV: Last Christmas)

In an alternate timeline where River Song refused to kill the Eleventh Doctor on 22 April 2011 at 5:02, the time kept alternating between 57 and 58 seconds past the minute. (TV: The Wedding of River Song)

Behind the scenes
In the cinema introduction to The Day of the Doctor, the Eleventh Doctor claimed that "all 57 Doctors" would appear in the 100th anniversary special of Doctor Who in 2063.