There is no fish per inch rule that is relevant in saltwater. In a 80g tank you could have 3 fish or 10 fish, all depends on their size, tempermeant, swim space and how much they contribute to the bio load ie: pooping etc.
What type of fish did you have in mind?
180g- Mostly BIG fish and some coral. ~80g Nuvo- My coral tank with "happy fish"~ 90g- FOWLR Not the not happy type of fish~ 125g- Freshwater Malawi Cichlids ~10g- Nuvo- The refugees from the Ich of '18