Fatto generale: 21743 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21743 is an unremarkable number.
Trivia: 21743 is a boring number.
Data storica: July 12th is the day in 1918 that the Japanese Imperial Navy battleship Kawachi blows up at Shunan, western Honshu, Japan, killing at least 621.