3
The enrollment, being taken primarily for military purposes (compare Numbers 1:3, Numbers 1:20), would naturally be arranged by hundreds…

Of the children of Manasseh, by their generations
(See Gill on Numbers 1:20).

The people were numbered to show God's faithfulness in so increasing the seed of Jacob, so that they might be better trained for the wars and conqu…
Albert Barnes
AlbertBarnes