Numbers 1:19 (KJV)

And Moses and Aaron took these men which are expressed by their names:

And they assembled all the congregation together on the first day of the second month, and they declared their pedigrees after their families, by the house of their fathers, according to the number of the names, from twenty years old and upward, by their polls.

As the Lord commanded Moses, so he numbered them in the wilderness of Sinai.

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.