Numbers 1:18

1:18 and they assembled the entire community together on the first day of the second month. Then the people recorded their ancestry by their clans and families, and the men who were twenty years old or older were listed by name individually,

Numbers 1:20

1:20 And they were as follows:

The descendants of Reuben, the firstborn son of Israel: According to the records of their clans and families, all the males twenty years old or older who could serve in the army were listed by name individually.