Numbers 1:18

18 and on the first day of the second month they assembled the whole congregation together. They registered themselves in their clans, by their ancestral houses, according to the number of names from twenty years old and upward, individually,

Numbers 1:18 Meaning and Commentary

Numbers 1:18

And they assembled all the congregation together on the first
[day] of the second month
The month Ijar, as the Targum of Jonathan, answering to part of April and May: this was done on the selfsame day the Lord spake unto Moses about this affair, ( Numbers 1:1 ) ; so expeditious were he and Aaron in doing the will of God:

and they declared their pedigrees;
either Moses and Aaron, according to Aben Ezra, who inquired when they were born, because of the computation of twenty years; and then their birth was wrote down, as he says; or rather the people declared their pedigrees, of what tribe, family, and house they were, who their parents, when born, and so, of course, how old they were; Jarchi interprets it, they brought the books of their genealogies, and witnesses to confirm the birth of everyone of them, to show their genealogy according to their tribe; nor is it at all unlikely that every family and house, or master thereof, kept a register of those born to him in it, whereby their age could be ascertained as well as pedigree:

after their famines, by the house of their fathers, according to the
number of their names, from twenty years old and upwards, by their
poll:
that is, every tribe gave an account of the families in it, every family what houses were in it, and every house what number of males were in it, and of what age; and such were numbered who were twenty years old and upward.

Numbers 1:18 In-Context

16 These were the ones chosen from the congregation, the leaders of their ancestral tribes, the heads of the divisions of Israel.

17 Moses and Aaron took these men who had been designated by name,

18 and on the first day of the second month they assembled the whole congregation together. They registered themselves in their clans, by their ancestral houses, according to the number of names from twenty years old and upward, individually,

19 as the Lord commanded Moses. So he enrolled them in the wilderness of Sinai.

20 The descendants of Reuben, Israel's firstborn, their lineage, in their clans, by their ancestral houses, according to the number of names, individually, every male from twenty years old and upward, everyone able to go to war: