In the wild, the number of garter snakes that can be found on one location, is not dependent on how social they are, but how much food there is available.
The only "social" event they know is when they seek a partner for reproduction-reasons (Hi babe, do you come here often? Let's copulate! Was it as good for you as it was for me? Ok, I'm gone now...) and when they are "forced" to use the same place to brumate.