Fatto generale: 21553 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21553 is a boring number.
Trivia: 21553 is an unremarkable number.
Data storica: January 3rd is the day in 1997 that China announces it will spend US$27.7 billion to fight erosion and pollution in the Yangtze and Yellow River valleys.