NET

20 And they were as follows: The descendants of Reuben, the firstborn son of Israel: According to the records of their clans and families, all the males twenty years old or older who could serve in the army were listed by name individually.

Darby

20 And the sons of Reuben, Israel's eldest son, their generations^D932, after their families, according to their fathers' houses, by the number of the names, according to their polls, every male from twenty years old and upward, all that went forth to military service:

Darby Bible Notes

generations - As Gen. 2.4, throughout this chapter.

KJV

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;