Numbers 1:3

“From twenty years old and upwards, etc.] All that had entered into their twentieth year, or, as it should rather seem, who were full twenty years of age, and all that were above it without any limitation; though some limit it to fifty, and others to sixty years, when men may be reasonably excused going to war; for to know who were fit for it seems to be a principal design of this order, as follows: all that are able to go forth to war in Israel; who being about to journey, might expect to meet with enemies, with whom they would be obliged to engage in battle; and therefore it was proper to know their strength, and whom to call out upon occasion: Aben Ezra observes, that the phrase “in…”

“Aaron shall number them by their armies--or companies. In their departure from Egypt they were divided into five grand companies (Exo 13:18), but from the sojourn in the wilderness to the passage of the Jordan, they were formed into four great divisions. The latter is here referred to.”

“A month had passed away since the setting up of the tabernacle Exodus 40:2, Exodus 40:17 : and the Sinaitic legislation was now complete (compare Leviticus 27:34). A census (“sum”) was commanded, to be based not upon any fresh registration of individuals, but upon that which had accompanied the previous collection of the offerings. Compare Exodus 30:11, etc.; Exodus 38:25-28. The offerings had been probably tendered by the people in groups, and if certificates of registration were furnished to such groups, the new census might be easily carried out by means of these documents, and got through Numbers 1:18 in a single day. The present registration enrolled persons “after their families, by…”

“twenty: Num 14:29, Num 32:11; Exo 30:14 able: Num 26:2; Deu 3:18, Deu 24:5; Sa2 24:9; Ch2 17:13-18, Ch2 26:11-13 by their: Num 33:1; Exo 12:17 Numbers 1:4”