General fact: 21086 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Mathematical fact: 21086 is an unremarkable number.
Trivia: 21086 is an unremarkable number.
Historical date: September 23rd is the day in 1641 that the Merchant Royal, carrying a treasure worth over a billion US dollars, is lost at sea off Land's End.