John Calvin Commentary Numbers 26:51

"These are they that were numbered of the children of Israel, six hundred thousand and a thousand seven hundred and thirty." — Numbers 26:51 (ASV)

These are the numbered of the children of Israel. By this sum total, what I have previously referred to is more clearly shown: that amid so many losses, and especially after the terrible vengeance which God had recently executed, the race of Abraham was preserved in an incredible manner, so that the fulfillment of the promise might not be realized for only a small body of persons.

Nature itself and reason would have suggested that only a few should enjoy the promised land; but if the inheritance had been restricted to a small number of men, God’s promise would have lain, as it were, in obscurity and concealment. Yet within thirty-eight years, during which more than 603,000 men had fallen, God marvelously brought it about that the same number of persons should still remain, except for only about 2,500.

Assuredly, they must be blind four times over, as it were, who do not behold in this bright mirror God’s wonderful providence, and the faithfulness of His gratuitous adoption, and His steadfastness in keeping His promises. At the same time, what I have already referred to in Deuteronomy also clearly appears: that those who survived were strikingly admonished by this great loss, that they should not at any time fall away into superstition.