1/5 is good rule or guide for reef. For fish only with L/R ( at least 1 lb. per gallon)and L/s( at least 3 inches), after up and running with no problems for 1 year can go as high as 1/3 or even 1/2.
My opinion of course. Others may disagree. Again only a guide.
Usally inverts: snails, crabs, shrimp, etc. are not included in the "rule of thumb" measurement because they hardly contribute to the "bio-load". So you could have 11 - 1" fish with the addition of inverts IMO