Numbers 1:18

18 and on the first day of the second month they assembled the whole congregation and recorded their ancestry by clans and families, counting one by one the names of those twenty years of age or older,

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 men were appointed from the congregation; they were the leaders of the tribes of their fathers, the heads of the clans of Israel.

17 So 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 and recorded their ancestry by clans and families, counting one by one the names of those twenty years of age or older,

19 just as the LORD had commanded Moses. So Moses numbered them in the Wilderness of Sinai:

20 From the sons of Reuben, the firstborn of Israel, according to the records of their clans and families, counting one by one the names of every male twenty years of age or older who could serve in the army,