You could probably keep a school of a dozen Exodon Paradoxus in a 40 gallon, as long as they are kept well fed, with a riverine environment, and no other species of fish in the tank.
Be prepared for a lot of work, but some enjoyable feeding sessions.
The Bucktooth Tetra (Exodon paradoxus) grows to around 4.5 or 5 inches. A set up 5 gallon (nominal) tank would only hold a maximum of 4 gallons. (No water in the top 1inch and water displaced by gravel substrate at bottom). So using the basic Aquarists rule of "1 inch of fish needs a minimum of 1 gallon of water". Even pushing the limits, the tank is not big enough to house a Bucktooth tetra. The recommended tank size for them is around 30 Gallons.
Dpends on the size of the fish its one inch full grown for every gallon
A betta fish would be much more suited to a 1 gallon tank than a 'variety'. You shouldn't keep much more than one fish per gallon anyway.
2 or 3
The general rule for stocking a fish tank is 1 inch of fish per gallon of aquarium water.
The rule of thumb is to keep no more than 1" of fish per gallon.
No matter what its dimensions are it is way too small to keep most fish in successfully.
No, these fish can grow past 1.5 ft in length
"fish bowls" are not designed for fish regardless of how they are marketed; you should not keep fish in anything smaller than a 10 gallon tank.
The only fish you can keep in a 5 gallon tank is one betta (Siamese fighting fish) on it's own as that is a very small tank.
It may be possible to keep 4 small fish in a one gallon fish bowl, depending on the type, size and needs of the fish you choose. It will also depend on whether or not the fish bowl has a filter, and if your fish can live without a heater. For example, because they do not get large, 4 white cloud minnows can be kept in a one gallon bowl with a filter and they do not require a heater. You can also keep a male beta or Siamese fighting fish with a few white cloud minnows; or 2 small goldfish which can grow too large or the bowl.
If the tank is large enough. For rearing young Oscars a 50 gallon tank is about right for 5 fish.