According to the Thirteenth Doctor, 51 was a pentagonal number. When she asked Yasmin Khan to pick a random number, Yaz chose 51. The Doctor set a bomb to go off early, in 51 seconds.
The Doctor favoured the number 51. She revealed that it was also the atomic number of antimony, and the number of the Federalist Papers written by Alexander Hamilton. (TV: The Tsuranga Conundrum)