How many fish and inverts can you keep in a 20 gallon saltwater reef aquarium?

This is the rule I use for my 15 gallon saltwater tank. Group A: Basslets, Gobies, and Jawfish. Group B: Firefish, Cardinal fish, Blennies, Chromis(pairs), Anthias, and Clownfish(pairs). You can pick 2 fish from Group A(can be different species, it should be different species) and 2 fish of 1 species from Group B. Plus you can add up to 3 damsels or snails or shrimp or cleaning Seastar.