One developer or sysadmin running an instance can support hundreds of non-technical users per server. This is a model that's already validated by mastodon, which has >1M registered users across thousands of servers (and ~40k weekly active users)
Mastodon will need to find business model sooner or later as internet isn't free as in beer because beer costs money and you can't handle 100m users with donations.
As long as the members are part of several disparate groups, that's probably more of a good thing. Closer, tighter communities.
Also, I thought the bandwidth came from the users, since it's decentralized?