I know in a community setting you have more passive members than not. I just don't know if I want to start out enabling that before having really solid admin and moderation tools.
The other thing I noticed that in let's say "stable" communities (slow growth, many long term members) there is much more of self-policing going. If someone acts outside of the implicit or explicit rules there is bigger chance someone would respond to that in "hey, behave, we don't do that here" comment and 9/10 that's enough, no mod involvement is needed, no bans or other punishment needs to be applied.
When community is a lot of "random people off the street" (no effort to create account or join, just start writing) that happens much more rarely
"Dunbar's number is a suggested cognitive limit to the number of people with whom one can maintain stable social relationships—relationships in which an individual knows who each person is and how each person relates to every other person. This number was first proposed in the 1990s by British anthropologist Robin Dunbar, who found a correlation between primate brain size and average social group size. By using the average human brain size and extrapolating from the results of primates, he proposed that humans can comfortably maintain 150 stable relationships."
many groups have a few active people and many less active people. one group i am in has about 80 members, 10-20 or which are active and keep a steady stream of communication. if the group were to grow much beyond that, it would be to much.
so you are right that 20 active people is a good group size but if there is a technical limit of 20 that means i have to police all the inactive people in order to have room for active people. seems to create more overhead than i'd be comfortable with.
wechat groups have a limit of 500 people. it's big enough to allow some flexibility for groups that have lots of members but little communication, but don't allow the groups to get so large as to get out of hand. the ideal technical limit for me is somewhere between 100 and 500. less than that and i'd run into to many corner cases because some group somewhere hit the limit. most of the groups i am active in have less than 100 members, but many have more than 20, but i am also in some groups that have more than 200 members, most of which are just reading and a few contributing.