On another forum:

With the Flood at about 4,500 years ago, it needs less than 0.5% per year growth. That’s not very much. I think today's rate is about 1.7%

In the next generation after Noah, Shem had 14 grandsons, Ham, 28 and Japheth, 23, or 130 children in total.

The Bible records longer life spans, so people would have been fertile longer, possibly having several 'families' over their life span.

Population growth is exponential.

Take the Duggars. They have 19 children. Let's suppose that each child has 3 children, which is pretty conservative. That will be 57 grandchildren. Then suppose that this trend continues. Some children may have no offspring and some more, but let's say they average only 3.

57x3=171

171x3=513

513X3=1,539

After that the numbers start to go up rapidly. exceeding hundreds of thousands in just a few more generations.

If there were 300 million people in the world at the time of Christ’s Resurrection, this requires a population growth rate of only 0.75% since the Flood, or a doubling time of 92 years—much less than the documented population growth rate in the years following the Flood.

The Jews are descendants of Jacob (also called Israel). The number of Jews in the world in 1930, before the Nazi Holocaust, was estimated at 18 million. This represents a doubling in population, on average, every 156 years, or 0.44% growth per year since Jacob. Since the Flood, the world population has doubled every 155 years, or grown at an average of 0.45% per year. There is agreement between the growth rates for the two populations. Is this just a lucky coincidence?

http://creation.com/where-are-all-the-people