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;
*The verse says: The descendants of Reuben, who was Israel's firstborn, are listed based on generations, families, and father's houses. This count includes all adult males aged 20 and above who could participate in wars.