Numbers 1:20-21

And the children of Reuben, Israel's eldest son, by their generations, after their families, by the house of their fathers, according to the number of the names, by their polls, every male from twenty years old and upward, all that were able to go forth to war; Those that were numbered of them, {even} of the tribe of Reuben, {were} forty and six thousand and five hundred.