The worst so far is 30 fish in a 20 gallon tank and the person didn't know they had to do water changes
. Apparently they had that tank for two years.
Sounds a lot like my old neighbor... except she had 36 fish in one, including, among many others, cichlids, guppies, a betta, a bluegill, tetras, a 6-inch synodontis catfish, 2 plecos, and at least a dozen big goldfish 5 inches plus.
Personally, I keep three tanks - 2 planted five gallon ripariums with male bettas and zebra nerite snails and then I just started a twenty gallon riparium that so far has just a betta and a couple of red cherry shrimp but which will eventually also have some other fishes in it - probably a big school of pygmy cories with a few others.