There isn't much in the way of a max amount of people. Internally the game uses a 4 byte int. So you could theoretically have up to 4,294,967,295 people. Although, I wouldn't be surprised if some of the formulas used in the game start breaknig down before then due to rounding or overflow errors. When you set a player to an AI, they get a fixed amount that's fairly large. (I can't remember the value anymore, but it was something like 16384 or 65536). At any rate, it's large enough to not really be an issue other than feeding them. (But AI players don't have to feed their people).
Edit: Correction. That's not "people", that's kids, workers, or scientists taken individually. So 4,294,967,295 kids, or 4,294,967,295 workers, or 4,294,967,295 scientists. But if your total pop grows over that, overflow error should cause a value less than that to be reported for your total population.
