Fatto generale: 9865 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 9865 is the number of digits in the 15^{th} Fermat number.
Trivia: 9865 is an uninteresting number.
Data storica: January 3rd is the day in 1815 that Austria, the United Kingdom, and France form a secret defensive alliance treaty against Prussia and Russia.