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?

If it’s about self sustaining a group, also have a look at some guidelines I wrote a decade ago on what makes a community of practice work (based on Etienne Wenger‘s work).

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.

Replied to a post by Chris M.Chris M.

What size do you consider “critical mass” for an active community centred around a niche hobby/topic?