According to Tom Pickett on this site: http://www.ldolphin.org/pickett.html
Although it is difficult to obtain an actual value of world population at the time of the flood, 5 to 17 billion people would appear to be reasonable populations, with an average of around 10 billion.
The site shows the assumptions and formula used for calculation. Inherent in the assumption of an average age of 900 years is a tacit assumption of a very low mortality rate. On the other hand, it has what could be considered a very low average family size given the reported ages of the pre-diluvians (6 to 7 children per family for the figures given although other family sizes are considered). In view of these figures, the following comment seems highly pertinent:
If the population reached over a billion, there would tend to be some logistical problems in feeding and caring for the population (clothing, housing, jobs, etc). This indicates that they would have required a higher level of technology than what we currently give them credit for.
Given this, it seems likely that the pre-flood population was approaching the carrying capacity of the accessible arable land and that at least some of
...how great the wickedness of the human race had become on the earth, and that every inclination of the thoughts of the human heart was only evil all the time. - Genesis 6:5 NIV
may have been triggered by conflict over scarce resources.