I'm not aware of any limit per se to the size of a group, however the real issue I'd be looking at is would you really want to be a member of a group of that size.
A large group of 10-20+ shares splits the bills more but can create problems with availability as there are so many people who want access to the a/c. This can create tension and also personalities/politics can come into play, causing infighting. Groups of that size aren't really syndicates, they're really mini flying clubs.