Numbers 1:2

Houses. The families consisted of the immediate descendants of the 12 patriarchs; the houses were from the subdivisions of these. The same plan of numbering the people was adopted on other occasions, Josue viii. 16., and 1 Kings x. 20. --- Sex, between 20 and 60 years of age. All the subjects of the Eastern kings may be called upon, if they be able to bear arms; and hence we find such immense armies in the Scripture, and in profane history. Moses numbered the people once before, (Exodus xxx. 2,) and found exactly the same number of warriors, the dead being replaced by others, during the space of seven months. Perhaps the odd numbers might not be specified, as all the totals consist of so many exact hundreds, except that of the tribe of Gad, ver. 25. On the former occasion, the people were not perhaps ranged according to their tribes, which was now deemed necessary, as they army was going to begin its march under its respective leaders. (Calmet)