Currently, in 2017, the World's population is estimated to be, 7.5 Billions. Growing at a rate of 1.11% per year, it could reach, if maintained, 11.2 Billions by the year 2100.
Using a same growth rate of 1.1% per year, the World's population before the Biblical Flood of Noah, would have been a minimum of 147 Millions, and a maximum of almost 20 Billions if a growth rate of 1.4% is used.
Considering the Life Spans of our Pre-Flood ancestors, the 20 Billions is the more likely number.
In the very beginning of the Bible history, we have examples of thriving populations, if we consider that (Genesis 4):
- Able was a keeper of sheep. (Animal herder)
- Cain was a tiller of the ground. (Farmer)
It is very unlikely that Adam, Eve, and Cain and Able, were the only ones on Earth, or they wouldn't have had any necessity for animal Herding and Food production.
One must also note that after Cain killed Able, and went his way, he built a city, after his wife conceived. To build a city, again, Cain was not alone with his wife, for this to be endeavored.
Below, a very interesting presentation that deepens the above subject.