If we have zero population growth, and a generation is 20 years, every 200 years will multiply the number of descendants by more than 1000.
If cousins don't marry, there will be 1 million descendants in 400 years and 1 billion descendants in 600 years.
Yes, I know 1 billion descendants sounds high. Those cuddling cousins should lower the number - but not by more than 99.9%. So by 600 years, you can be pretty sure you'll have millions of descendants.
DEFINITION: Descendants are those that come after you..........Ancestors are those who walked before you.