Hecho general: 9921 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Hecho matemático: 9921 is an unremarkable number.
Trivia: 9921 is an unremarkable number.
Fecha histórica: February 28th is the day in 1993 that Bureau of Alcohol, Tobacco and Firearms agents raid the Branch Davidian church in Waco, Texas with a warrant to arrest the group's leader David Koresh.