20 (number)

20 was a number.

As a defence mechanism for the Doctor's TARDIS there were twenty wrong holes within the lock and one right one. Attempting to open the door through a wrong one would cause the lock to melt, preventing access entirely. (TV: The Daleks)

Behind the scenes
According to NOTVALID: The Last Dalek, a video game based on the events of TV: Dalek, Adam Mitchell was 20 years old in 2012.