Grand Theft Auto 5 - Los Santos Civilian Population?
While I cant find any official sources I'm going to take a stab at it.
The short answer is that there is no answer.
Here is how it works; you drive into a street and the game decides there should be pedestrians there. It spawns ten people with randomized clothes. You drive past them all and when gone far enough the game simply deletes them again. This is because of the limited memory, simulating every citizen all the time is currently not possible.
So there isn't a fixed "pool" of people, each one individually created by a designer. You can't make person #1003 be named Joe, wear black jeans that live here and drives that car. Or, I suppose you could but these need to be stored somewhere (hard drive or disc) space that the game already pushes to the limit. And for what gain? You simple drive past (or murder) the vast majority of them anyway.
The exceptions are of course the scripted events and missions, which contain specific characters that act in a specific way according to the story.
Now what we can do is extrapolate. Knowing the type of houses in a neighbourhood, the average amount of people living in those houses and the size of the neighbourhood we can calculate a plausible number of citizens (fun challenge, anyone up for some math?).
So in short, any number you would get wouldn't mean anything. The game is made to quickly populate any area with randomized citizens. Only the part of town in close proximity to the character you play actually exists.
Well my English is not great but I will try; there are 9.800.000 people living in L.A. county with a somewhat similar geography. It covers 4.083 square miles. GTA V covers 49 square miles. If you divide 9.800.000 by 4.083 you get 2400 (a little bit more). 49 times 2400 = 117.600. But offcourse this is just an estimate.
I've made a real scale comparison of the two. Los Angeles and Los Santos in one map.
No need for calculations!
According to the Grand Theft Auto Wiki, Los Santos has a population of over 4 million.