21 (number)

21 was a number.

Jo Grant once stated that three sevens were equal to 21. (TV: The Claws of Axos)

References in invalid sources
According to the Doctor's Five Hundred Year Diary, it was 'a fellow ape descendant, Douglas Adams, who had asked for [John] Lloyd's help in scripting [ Doctor Who], when a series of missed deadlines thudded past him... Much like a volley of tennis balls indicating a player will progress no further than number 21 in a professional body's ranking system'. (NOTVALID: Tom Baker stars in John Lloyd's lost Doctor Who adventure, The Doomsday Contract)