Fatto generale: 6406 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 6406 is the number of permutations of 8 elements where every cycle has equal length.
Trivia: 6406 is an uninteresting number.
Data storica: July 15th is the day in 1888 that the stratovolcano Mount Bandai erupts killing approximately 500 people, in Fukushima Prefecture, Japan.