Numbers 1:20

20 From the descendants of Reuben the firstborn son of Israel: All the men twenty years old or more who were able to serve in the army were listed by name, one by one, according to the records of their clans and families.

Read Numbers 1:20 Using Other Translations

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;
The people of Reuben, Israel's firstborn, their generations, by their clans, by their fathers' houses, according to the number of names, head by head, every male from twenty years old and upward, all who were able to go to war:
This is the number of men twenty years old or older who were able to go to war, as their names were listed in the records of their clans and families :