Bamidbar 1:18

18 And they assembled kol HaEdah together on the first yom of the second month, and they declared their family after their mishpekhot, by the bais of their avot, according to the number of the shmot, from twenty years old and upward, one by one.

Bamidbar 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.

Bamidbar 1:18 In-Context

16 These were the called men of the Edah, nasiim of the tribes of their avot, rashei alfei Yisroel (head-men of the clans of Yisroel).

17 And Moshe and Aharon took these men which had been designated by their names;

18 And they assembled kol HaEdah together on the first yom of the second month, and they declared their family after their mishpekhot, by the bais of their avot, according to the number of the shmot, from twenty years old and upward, one by one.

19 As Hashem commanded Moshe, so he counted them in the midbar of Sinai.

20 And the bnei Reuven, Yisroel’s bechor, by their toldot, after their mishpekhot, by the bais of their avot, according to the number of the shmot, one by one, every zakhar from twenty years old and upward, all that are ablebodied to serve in tzava;