Genesis 5:19

"And after he became the father of Enoch, Jared lived 800 years and had other sons and daughters."  Genesis 5:19

By now, if each man lived a millenia and had even waited to start having kids until he was in his sixties and kept at it even if only for a 100 years, there could have been a lot of people on Earth at that point.  If Adam and each of his sons had 20 sons there could have been 60 million people in Enoch's generation alone.  The next generation would have been 1 billion and the next 25 billion.  There are over 7 billion people in the world now.  If livescience is right that the Earth can only support 10 billion people max, then my guess is either they didn't have that many kids each and just like today there were people that didn't have kids.