Ruffian is correct, rule of thumb for a fish tank is always 1" of fish per 1 gallon of water.
We have a 140 gallon predator tank with a 5" black arrowana, 3 Florida Spotted Gars (1 is 6" and the other 2 are 9"), 5" Odoe African Pike, 4" Peacock Bass, 4" Svenii Pike Cichlid, and a 4" Siamese Gold Tiger Datnoid. And we still need to upgrade to a bigger tank in about a year.
Adding on to that rule of thumb, goldfish usually require about 1" per 2 gals. due to their high waste output and it's usually recommended for African cichlids to give them 1" per 5 gals. due to their territorial tendencies.
However, many stores, fish breeders and keepers, and hobbyists threw out the whole inch per gallon rule back in the 1980s because there are too many variables. Ex. A 10 gallon tank could easily hold 10 x 1" Neon Tetras but not 1 x 10" Oscar.