It depends on what the fish is. Danios, tetras, glofish, and catfish are schooling fish which means you need at least three of them or most likely they will end up dying. Moll
…ies, Platys, and guppies are not schooling fish which means it is ok to only have one of them and if you have more than one you have to remember that it is 2-3 females for every male you have in the tank because the males will harrass them. Cichlids do better by themselves but you can have them with other Cichlids of the same size that get to be the same size as them except for a few of them like flowerhorns which will attack each other. Goldfish are ok by themselves but you want to have at least a 20 gallon tank for one and a 30 for two. Semi-aggressive fish like tiger barbs and some catfish need to really only be with other semi-aggressive fish. Male Betta fish CANNOT be with other male betta fish because one will kill the other but you can usually put them in a tank with tropical fish or female bettas and be fine. (MORE) 
