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.