A lot of community types just simply don’t work without a minimum critical mass of members.
Imagine asking a programming question on a software development community of just 5 people. You end up with 3 people who aren’t active enough to see the question, 1 person sees but doesn’t have an answer and doesn’t respond (classic lurker), and one person sees it and responds that they don’t know the answer. Now imagine a community of 5 thousand people…it’s suddenly much more feasible to even bother asking the question.
Sure, fediverse could exist with just 5 people, but it would be worthless and pointless.