I guess you could get two but they are very territorial so don't be surprised if you see them fighting
you can put a betta in a 5 gallon tank
you can put a betta in a 54 gallon tank
No you should not 2 bearded dragons should be put in at least a 75 gallon tank. You could put them in a 55 gallon tank but they would be extremely stressed out.
either a 15 gallon tank or a 20 gallon tank
You can put it on a table if you choose. NEVER put a tank on the floor.
Sure, you can put it in a 20-gal. tank. It will probably do fine unless you put other fish in the tank that might eat it. The more water, the better for the fish. It may not look particularly aesthetic to have a little, bitty fish swimming around in a tank that size but, that isn't your concern, I presume.
one
normal rule for fish in a tank is: For every 1 inch of fish there is 1 gallon of water
3
No
Yes.
In that 150 gallon fish tank try to put fish and maintain it