Fatto generale: 21747 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21747 is an uninteresting number.
Trivia: 21747 is an unremarkable number.
Data storica: July 16th is the day in 1915 that Henry James becomes a British citizen, to highlight his commitment to England during the first World War.