According to Gen 46:27, Jacob entered Egypt with a family of 70. Kohath was Levi's son, Amram Kohath's, and Moses Amram's, so Moses's generation was 2 generations from Kohath's. 600,000 men, with women and children, left at exodus (Ex 12:37).
Meanwhile, before Moses was born, pharaoh ordered, that all male children from then on have to be killed. Moses survived miraculously, so 600,000 has to refer to men from generation above. Roughly estimating, 35 women (half of 70 that entered Egypt), gave birth to 1,200,000 people, which gives a fertility rate of about 34,285 (assuming 0 mortality). This seems quite large. Moreover, if we assume that Jews didn't stop procreating with pharaoh's command, at that fertility rate, there would be about 20 billion women in Moses's generation.
Further, despite lacking partners from same generation, they could be impregnated by their uncles (Moses's mother was the aunt of his father, so it probably wasn't that uncommon), which would push the number even further.
Such numbers are very unlikely, and since Bible is inerrant, can you help me find error in my reasoning?