General fact: 21752 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Mathematical fact: 21752 is an unremarkable number.
Trivia: 21752 is a boring number.
Historical date: July 21st is the day in 1972 that Bloody Friday bombings by the Provisional IRA around Belfast, Northern Ireland – 22 bombs are detonated, killing 9 and seriously injuring 130.