I think it depends on what game system, and what style of group, and how the GM organizes the "seats". Some have a larger membership pool, but only a certain limited number of members sit to play together on any given day or session. Some games may actually have a lot of people, I've done that for a holiday game and Roll20CON gaming convention. Some have former players still listed in the group, that aren't actively playing and had to drop out, but they didn't kick them out of the Roll20 Players list. It's just up to people to communicate, so you can always ask the GM why there are so many, or how it works for them. Likewise, of course, your choice, if you want to be in a 4-person game, you can look for those or ask the GM if a game can be kept to that.
