If you have the right mechanics, you can increase group size significantly.

in our RP, we've got three GMs, over thirty characters and multiple locations and plots. The player count is lower, of course.

We've got an intricate timestamp system to make sure that characters generally don't get left behind, or if they do, that they can catch up.

If you've got multiple factions for players in your RP, you can keep it balanced by not letting any single faction grow too powerful.