Community vis-à-vis Forum
I think of a community as a state (-ity) of having a purpose in mind (mmun->mean) together (co-), not as an endurable space. I think of a forum as an endurable space, as a doored (from the Latin fores, i.e. door) space of focus (from the French foyer).
How many makes a community? I don’t know. I won’t pretend the Hebrew minyan actually shares etymology with community, but it does helpfully suggest a quorum of ten. How many is too many? Because a community is a purpose-coherent social state, Dunbar’s number suggests a “knee of the curve” of roughly 150.
It seems hard for a set of people to sustain a state of having a purpose in mind together. If the purpose can be fulfilled, then if it is, that set of people can attempt to self-herd themselves to an adjacent or follow-on purpose, and thereby “evolve” “the” community into a different community, a different state of having a purpose in mind together.
A forum can serve a community. If that community evolves, i.e. shifts coherence of purpose, the forum may be sustained if both the pre-shift and post-shift purposes benefit from similar-enough kinds of focus.
It seems that a forum sometimes is made to endure despite the dissolution of the community that motivated its origination as a shelter for focus. I can’t help but think of the chartered corporation as typically being such a forum. The first such charters were granted to incorporate a set of people to sustain, via a shelter for focus, a state of having a purpose in mind together of constructing a particular railroad.