Fatto generale: 638 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 638 is the number of fixed 5-kings.
Trivia: 638 is an unremarkable number.
Data storica: September 29th is the day in 1789 that the United States Department of War first establishes a regular army with a strength of several hundred men.