It all depends on what you are going for FO, FOWLR, Reef. A hex does not have as much surface space as a rectangualar or bow front tank. I would say about 3-4 fish. I have 4 in my 45 gallon and I think I am pushing but I also have corals so that puts a strain on how many I can have. HTH
probably 3-4 or maybe even 5 smaller fish, depending on the species, but in a 40 gallon, i would not have anything large like a tang, only smaller fish int he 3 to 5 inch(at maturity) range
General rule of thumb is 1" for every 5 gallons. Just a rule of thumb though. Go with what will make you happy. Research what kind of conditions the fish need. And it's usually obvious what overdoing it is. SW fish need more room the FW fish, just remember that in case you wanted to compare.