While is very hard to answer quite a question,and like mizipzor answered, its pretty much impossible to know how many people there are. "Pretty much impossible" so its possible, if you get my meaning.
After seeing this question 9 days ago I have been studying the area of San Andreas and made an estimate. I started with Los Santos. I looked at how many houses there are in Los Santos and, since the average American has 2.2 children (dont know how that works) I said okay, put 4 people in each house, 2 children, two adults. The number is 2,346 people. Keep in mind that these are houses, not apartments, which is where most of the population live.
Did the same for apartments and residential skyscrapers and BOOM! Los Santos has a population of about 1,848,228 people.In my final estimate I bring it down by 2,500 because not everyone has children of course. Done the same to Sandy Shores but increased it because the average American "Redneck" as many so call them, have about 4 children each. So I estimated that there are about 14,994 people living in Sandy Shores. I did that for Paleto Bay, Grapeseed, Dingnitty Village, every settlement in the game. I finally concluded that there are about 3.634,870 people living in San Andreas. Rockstar says over 4 million people live there, but we can all agree that thats total bull s**t.
I also want to say that there is obviously more cops in San Andreas than people. Go into invincibility mode and start a cop killing spree. Those f**kers keep coming!
My town of Eastbourne has a population of 100,000 and its a hell of a lot smaller than Los SAntos. Look at a map of Eastbourne and than Los Santos. Its an illogical imbalance but we can all forgive GTA cant we? But there's your answer. 3,634,870 people live in San Andreas. I hope I answered your question and I helped many people. Thank you very much for reading and terra!