Fatto generale: 21691 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21691 is an unremarkable number.
Trivia: 21691 is a boring number.
Data storica: May 21st is the day in 1994 that the Democratic Republic of Yemen unsuccessful attempts to secede from the Republic of Yemen; a war breaks out.