The nation’s first census was taken in the wilderness of Sinai in 1512 B.C.E., the second year of their coming out of Egypt, at which time they had their descent acknowledged “as regards their families in the house of their fathers.” (Num. 1:1, 18; see also Numbers, chapter 3.)
... The census at Numbers, chapters 1-3, indicates that there must have been many father-to-son generations during the 215-year stay in Egypt, the total number of men twenty years old and upward shortly after the Exodus being 603,550 (aside from the tribe of Levi).