Charles Ellicott Commentary Numbers 1:3

"from twenty years old and upward, all that are able to go forth to war in Israel, thou and Aaron shall number them by their hosts." — Numbers 1:3 (ASV)

From twenty years old and upward. —The result of the previous numbering (Exodus 30:12; Exodus 38:26), which was made about six months earlier and was probably obtained by counting the number of half-shekels that were paid, as Ithamar appears to have done (Exodus 38:2), exactly corresponds with the result of the current census (Numbers 1:46). However, the complete census, or the numbering and enrolment of persons according to their tribes, families, and fathers’ houses, appears to have been deferred until after the erection of the tabernacle, for whose construction the atonement money had been paid. If the whole was done in obedience to the command in Exodus 30:12 and regarded as one transaction, then only those who had already paid their atonement money would be numbered on the second occasion.

There is nothing impossible in the supposition that all of those who had been numbered six months previously were still alive. However, on this supposition, no allowance is made for the number of those who were below twenty years of age at the earlier period but had surpassed that age by the later period.

However, inasmuch as the sum total in both cases is divisible by ten, and inasmuch as the separate items in this chapter are given in tens (the smallest subdivision of the people adopted by Moses on Jethro’s recommendation, Exodus 18:21), no objection to the historical accuracy of both records can be sustained if it is allowed that the number of those who had reached the age of twenty years since the earlier census corresponded closely with the number of deaths during the same period. The entire objection, however, is removed far more satisfactorily by the supposition that there was only one census (see the Introduction).

By their armies. —A better rendering is their hosts or companies.