By their generations. — The toledoth, or generations, included the whole of the descendants of the head of the tribe (Genesis 5:1; Genesis 6:9).
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;