sorry, there is no rule, imo. it depends on your set up and what fish you get.
Example, if you have a good amount of lr, good filtrations, ect, you'll be able to have more fish. Your tank will tell you....but,
if you're fighting high nitrates, you may be overstocked.
Plus some fish like lots of swimming room and don't get along with others...some fish stay at the top, others stay at the bottom, some swim all around...
depends on your tank and what fish you want.