Numbers 1:19

17Moses and Aaron took these men who are mentioned by name.

18They assembled all the congregation together on the first day of the second month; and they declared their ancestry by their families, by their fathers’ houses, according to the number of the names, from twenty years old and upward, one by one.

19As the LORD commanded Moses, so he counted them in the wilderness of Sinai.

20The children of Reuben, Israel’s firstborn, their generations, by their families, by their fathers’ houses, according to the number of the names, one by one, every male from twenty years old and upward, all who were able to go out to war:

21those who were counted of them, of the tribe of Reuben, were forty-six thousand five hundred.