In the beginning, there were zero humans. Then 2 were created on the sixth day. After the fall, they had two sons, bringing the world population up to 4. Then one son killed the other, so it went back down to 3. This all happened at some time before the world’s 130th year (which is when the replacement son was born).

That’s what the Bible says, anyway. In reality, the world population around 4000 BC was probably at least 7 million.

After that, the Bible doesn’t tell exactly how many people there were. So I’m going to have to make some rough estimates to get an idea of how many people there would have been by the time of the flood. Here’s what we know (based on the numbers given in Genesis 5):