Matthew Poole and continuators on Numbers 1:20

20 And the children of Reuben, Israels 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.

Heb. to wit, their generations, i. e. the persons begotten of Reubens immediate Children, who are here subdivided into families, and they into houses, and they into particular persons.