The server can easily hit 30+ people if there’s a reason to.
Mostly what it boils down to (at least myself included), is why would you want to join the server if 10 people are online? What if you’re a cop main and noone is on cop?
When there’s 20 on, there’s usually enough on both sides for great interactions.
More wide-scale events could boost the server pop., but staff are people too, and the gmod & nebulous players are getting older, making it harder to find time to play. Some things are just more important, and with gmod being an old-ass game, not a lot of new players are comming. It way past its glory days.
TLDR; people are old