My personal sweet spot is 20-35, though I tend to go a little higher than that at times because I also dislike a stagnating playerbase.
There are tiers of activity, and usually 40ish players is 25 pretty active players, 8-10 semi-active players, and 5-7 barely-there players.
Fewer than ~15 active players and people can’t keep themselves busy. Staff work can actually go UP because less story happens organically, socially. 18-25 active players is a really, really good sweet spot for social RP providing enough drive to keep things rolling when plot is on a cool down, IME. It’s also big enough to provide lots of player GMs without anyone having to run constantly.
The top limit, though, is not actually about staff capacity. I would not raise it if I had more hands (in fact, I have a lot of hands!), because it’s not about how many people I can get on plots or provide story for.
It’s about the reasonable size of a community that functions well together, develops healthy norms, and builds interconnected personal relationships that contribute to a cooperative play space.
This is important to me for all kinds of reasons, including maintaining the trust for folks to feel comfortable reporting problems, paying enough attention that I can often be alert to their presence even before a report bubbles up, the ability to keep an eye on how story is spread, and generally just… having a feel for stuff.
Even with help, I can’t do that at more than 35-40 players (and it’s a big part of why I count by player and not character).
I’m not perfect at it by any means, but I know I get much, much worse at it when there start to be so many people on the game that I don’t actually know a good portion of them.
And I tend to think that a community where folks know each other is a more pleasant place to play for lots of other reasons, too. Players are more cognizant of much work their fellow players put in to GM for them. They know who’s having a tough time in RL or that weekends are rough for that person they’d love to catch . THEY are aware when something feels ‘off’, and are more likely to let staff know.