Jamieson, Fausset and Brown on Numbers 1:45-46

45-46. all they that were numbered were six hundred thousand, &c.--What an astonishing increase from seventy-five persons who went down to Egypt about two hundred fifteen years before [see on Ge 46:8], and who were subjected to the greatest privations and hardships! And yet this enumeration was restricted to men from twenty years and upwards [Nu 1:3]. Including women, children, and old men, together with the Levites, the whole population of Israel, on the ordinary principles of computation, amounted to about 2,400,000.