Limitation of the number of users in Private and Public world

I am creating a new world in development as private world and checking the performance when many users are in one instance of the world. Although the setting of world member capacity is set 20, more than 6 users cannot enter by “Invite” or “Join” to friend. I guess the possible reasons are Network limitation, or computer specs or whether the world is published or not, and no idea. Is there other limitation of the number of entry to private instance of the private world? Or does it differ in an instance of published world (public)?