Fatto generale: 21737 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21737 is an unremarkable number.
Trivia: 21737 is an unremarkable number.
Data storica: July 6th is the day in 1809 that the second day of the Battle of Wagram sees a French victory over the Austrian army in the largest battle yet of the Napoleonic Wars.