My current group has 15 members, six of which actually play regularily. There's some turnover - being scientists, every one of us will eventually spend some time abroad or even vanish to smewhere else for a job - but there also is a hard core who are almost always present. Then there're 'rim' players - who participate when they have the chance, are in town or can arrange with work.
Still, we only play irregularily, sometimes on two following weekends, sometimes with gaps as long as two months. We have set up a web site to cooordinate meetings, whcih is the most convenient way, as all of us have access to the internet.
We have three regular GMs, me included, and play characters in all three campaigns, which also take place on the same timeline - this is due to the fact that all of us like to play sometime, and noone wants to force anyone to GM all the time. The three plots aren't mingling though, and only the respective GM has an oversight about their plots.
Before that group, I was in two others: one from high school (guys from my year's courses, mainly - the group blew often, but hey, gamers were hard to come by back then). The group had 4 players and a GM max and played once a week at least. After that, I was in one of these high-turnover game shop community groups, whcih was totally different, kind of fun, but also killed any chance of character advancement and coherent NPC development because every session was GMed by soemone else.
I have a character whom I played trhough all three groups, btw, though she has been tuned down twice, after entering each new group. Meh. At least, that's a decent way to simulate SOTA advancement.