I remember the doomsayers in the 80s saying that by the year 2000 there would be 13 billion people. I'm not sure where but it was one of those things that was in the news one day as well as the paper and a couple of magazines.
I think extrapolating current population (at that time) and figuring out the projected population later wouldn't be very hard, so being off by that much just basically makes me feel no fear about a larger and larger population.
I think the scientists (or at least the environmentalists etc.) that talk about large populations as though it's a looming disaster do it in a way to make it look more dangerous than it really is.
For example, they probably look at how much food a city or country imports or grows for itself, then say "They only make enough to feed 5,000,000 now...they cannot feed more than that!"
I think they can, the people that order/grow the food will increase production if the population calls for it. They've already done that many, many times.
I think there can be an agenda even behind something like population. I think there's an environmentalist agenda based on guilt (for being an evil homo sapiens) behind this.
Check that old mistake "The Population Bomb" by Paul R. Ehrlich.