The 1 per X amount of gallons rule? saltwater reef tank?

im new at this i havnt even got my tank yet but the tank is already set up and im buying it with everythin included along with some fish. ive heard of a cleaning crew? my question is i might change some fish and i look at the "rules" for the fish and they all say 1 per 5 gallons or one pe 25 gallons...does this mean i can put as many fish in a tank as i want or can i only put a certain amount. for example..1 brittle starfish per 25 gallons hermit 1per5 peppermint shrimp1 per 10 emerald crab 1 per 25...does this mean i can have 1 emerald crab and 2 peppermint shrimp and 5 hermit crabs and 1 brittle star fish all in one tank along with whatever fish i want?