Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
A month had passed away since the setting up of the tabernacle Exo 40:2, Exo 40:17 : and the Sinaitic legislation was now complete (compare Lev 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 Exo 30:11, etc.; Exo 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 Num 1:18 in a single day. The present registration enrolled persons "after their families, by the house of their fathers;" and was superintended not by the Levites (see Exo 38:21 and note), but by Num 1:4 an assessor for each tribe to act in the business with Moses and Aaron. The purpose now in view was not religious only. The census now taken would serve as a basis for various civil and military arrangements.
The princes of the tribes, selected Num 1:4 under divine direction, were for the most part the same persons as those chosen a few months previously at the counsel of Jethro Exo 18:21-26. Nahshon, prince of Judah, is mentioned in Exo 6:23, and Elishama, in Ch1 7:26-27. The peers of men like these were no doubt entitled, among their fellows, to the epithet "renowned," Num 1:16.
The enrollment, being taken principally for military purposes (compare Num 1:3, Num 1:20), would naturally be arranged by hundreds, fifties, etc. (cf. Kg2 1:9, Kg2 1:11, Kg2 1:13). In eleven tribes the number enrolled consists of complete hundreds. The difference, in this respect, observable in the case of the tribe of Gad here Num 1:25, and of the tribe of Reuben at the later census Num 26:7, is probably to be accounted for by the pastoral, and consequently nomadic, habits of these tribes, which rendered it difficult to bring all their members together at once for a census. Judah already takes precedence of his brethren in point of numbers (compare Gen 49:8 note), and Ephraim of Manasseh (compare Gen 48:19-20).
When a census of the tribe of Levi takes place. Num 3:15; Num 26:62, "all" the males are counted from a month old and upward, and not, as in the other tribes, those only who were of age for service in the field.
had spoken - Render spake. The formal appointment is only now made, in reward for their zeal Exo 32:26-29, though reference to their future office appears previously in Lev 25:32 ff, and they had already acted as assistants to the priests (compare Exo 38:21).