Keep in mind the character tab might not be representing active characters; the post with the sheet will still be there regardless if a player is still active or dropped out.

It mainly comes down to how a GM wants to run their RP. Some, like myself, focus more on character driven plots and player engagement so having massive quantities of active players would be a nightmare to manage. Other times, the story itself calls for a very limited number of characters (a rough example off the top of my head, a RWBY RP where canonically teams are 4 people), and so on.

So yeah, it's common, but there's always a reason for it.