Stocking 75 gallon with sump

I just got a 75 gallon tank and I have a kole tang, a blue chromis, and a cleaner shrimp along with some snails/crabs. The tank has a 20 gallon sump so I was wondering how many fish I can put in the tank. I have heard of the "one fish per 10 gallons rule" but does the 20 gallon sump allow me to have 2 more fish on top of the 6-7 fish already? It might be a dumb question but I am new to this hobby! Thanks!