Albert Barnes Commentary Numbers 3:43

"And all the first-born males according to the number of names, from a month old and upward, of those that were numbered of them, were twenty and two thousand two hundred and threescore and thirteen." — Numbers 3:43 (ASV)

This result, when compared with the number of male adults (603,550, compare to Numbers 2:32), is small, as the usual proportion of firstborn sons to the total male population is about one in four. The explanation offered is that the law of Exodus 13:1-2 prescribed the dedication of only those who would be firstborn from that time forward.

On the other hand, this number is very large to have been born among two million people in a single year, and it must be admitted that some unusual causes must have been involved.

Such causes, not to mention the divine blessing, may be found in the sudden development of national energies that would immediately follow the Exodus.

Before that event, the people's miserable condition, and especially the inhuman order for the destruction of their firstborn, would have seriously limited the rate of marriages and births. This rate would naturally, when that limitation was removed, show a sudden and striking increase.