It depends I think. Critical mass towards what? Local agency? Go mainstream from a niche? Self sustainment of the group? What are your underlying assumptions?
My rule of thumb for ‘stable’ groups is about 8 people (say 5-12, but 12 is better for ‘learning’ groups and likely too big for coordinating ‘doing’ groups), and then an ‘uncomfortable’ zone up to 25 or so, where I feel there’s a new equilibrium from 25 to 35 or so (3 times ~8, the uncomfortable zone is more like 2 times ~8, with a risk of splitting in half)
Assuming you’re after a self sustaining group for a niche hobby, I’d say 25-35 people. Depending on a multitude of factors in practice, like frequency of interaction, geographic dispersal, all the stuff in the slides linked above.