By their generations That is, the persons begotten of Reuben's immediate children, who are here subdivided into families, and they into houses, and they into particular persons.
Related Commentaries of Numbers 1:20
Numbers 1:20
20 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;