Depends on the type of fish in the tank. Its argued several different ways ,the easiest to explain is 1 " of adult fish per gallon. But that has been proven incorrect by alot of fish keepers. Me personally i say african cichlids can be overcrowded but ,make sure you do at least 25% a week water changes and you have filtration rated for ten times the tank size.
Sa cichlids grow much larger on average so they should be less crowded . Bigger fish =more waste. for sa cichlid i take the length of the fish add it to th girth of the fish and times that by 4 . So a 10"l by an 8"girth fish x 4 =72 so i would keep that single fish in a 72 gallon or larger tank.