Fatto generale: 21764 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21764 is a boring number.
Trivia: 21764 is a boring number.
Data storica: August 2nd is the day in 1989 that a massacre is carried out by an Indian Peace Keeping Force in Sri Lanka killing 64 ethnic Tamil civilians.