This comes down to how often you are performing water changes. 

Think about how often you would like to perform the water changes and realistically could you keep on top of them as it could become a long tedious job if you overcrowd you tank.
 
The general rule is that saltwater fish require more space than freshwater fish. It is down to personal opinion on how much you put in the tank but always take into consideration the potential size of the fish not the size they are when you add them to the tank.