With something that large you could even add a small flowing water feature. As many people already said, the number of snakes you could put in there comfortably depends on many factors. In any case, you will need to keep an eye on the situation and watch for social stress. Garters aren't very prone to that, but it can happen. Sometimes certain garters will stress out (watch for nose-rubbing, constant obsessive rooting in substrate, trying to escape, etc.) when put with other snakes, sometimes it's just two particular snakes that don't like each other, etc. More than one basking area, plenty of hides, and visual barriers are helpful.

As far as introductions go, I've always just put groups together all at once. Then observe them over a couple of weeks. If nobody is stressing out, and they seem to be relaxed and comfortable then no worries. If you do see signs of stress, remove the ones that are stressing out and see how they do by themselves. I really don't think you'll have a problem putting 5-8 small-medium snakes in there. I've put that many in smaller tanks and they do just fine although it means more frequent cleaning and water/substrate changes.