Charles Ellicott Commentary Numbers 1:20

"And the children of Reuben, Israel`s first-born, their generations, by their families, by their fathers` houses, 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;" — Numbers 1:20 (ASV)

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).