John Calvin Commentary Numbers 31:37

"and Jehovah`s tribute of the sheep was six hundred and threescore and fifteen." — Numbers 31:37 (ASV)

And the Lord’s tribute of the sheep. The greatness of the victory is shown by the result, since such an abundance of cattle could only have been collected from a wide and populous country. It is probable that the land was not very fertile, and consequently only livestock, and not corn and wine, are enumerated among their wealth. Still, we may conjecture that it was famous for pastures, since barren mountains could not have fed so many oxen, goats, sheep, and camels.

Besides, it is most evident from the number of young women that the men who were slain were more numerous than their conquerors who had been sent to the battle. For if we suppose that each of them had an unmarried daughter, they would have outnumbered the 12,000 Israelites almost three times. Hence, again, it is manifest that the victory was effected by Divine power.

It may, however, seem strange that, although the nation was almost destroyed, their descendants nevertheless existed a short time afterwards, as if new Midianites had been begotten from the ashes of their fathers. For not a very long time elapsed between this slaughter and the time of Gideon, when they again dared to attack the Israelites voluntarily and, relying on their numbers, to rush into the very heart of Canaan. Indeed, they had already brought all the neighboring nations into subjection.

Since the Scriptures do not inform us how this could have happened, it only remains for us to conjecture that many of them, as often happens in a time of confusion, fled elsewhere and soon afterwards returned to the land, which was then unoccupied. For the sudden incursion of the Israelites was like a storm that soon passed away, nor was flight difficult for this unsettled and wandering nation.

It might also have been that many immigrants from various places flowed into the land when it was stripped of its inhabitants. Or perhaps the Israelites, having performed their work negligently, sounded the recall sooner than they should have, and God afterwards punished their negligence. At any rate, we are taught by this example that the wicked sprout up like foul and noxious weeds, so that, though often cut down, they soon cover the ground again.