Fatto generale: 21476 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21476 is a boring number.
Trivia: 21476 is an unremarkable number.
Data storica: October 18th is the day in 1967 that the Soviet probe Venera 4 reaches Venus and becomes the first spacecraft to measure the atmosphere of another planet.