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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 Joseph, [namely], of the children of
Ephraim, 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