Charles Ellicott Commentary Numbers 26:2

"Take the sum of all the congregation of the children of Israel, from twenty years old and upward, by their fathers` houses, all that are able to go forth to war in Israel." — Numbers 26:2 (ASV)

Take the sum ... — The same command had been given to Moses and Aaron (Numbers 1:2–3). In that case a man taken out of every tribe, the head of his father’s house, was appointed to assist Moses and Aaron in taking the census. It is probable that the same arrangement was made in the present instance, though it is not recorded.