I only saw the first 4 movies and some episodes of the TV-Series, so mabye this is mentioned in Genesys or some of the TV-Episodes or in a book.
When we see some scenes from the future, it is always about the resistance group of John Connor. There are several statemens that the human population is near extinction, I guess a big part died already because of the nuclear explosions seen in T3, but i can not remember an exact statement how many humans are still alive. Are there other resistance groups or not machine controlled areas on earth except John Connors Group in north america (Asia, Europe, etc...)? Is it ever mentioned or estimated how many humans are still alive on earth?
In Terminator 4 we see a little overview and can estimate the humans alive in Connors area, but I guess they are not more than some thousands, is this all that is left of human population in the future?