I wouldn't think you would need 20g for a DWARF puffer because they only grow to be 1". I would hink 2 or 3 would be good in a 10g, but don't put them with other fish. (I almost got some until I discovered that)
You could probably even get 4 for a 10 gal tank. They are fun little fish to watch. And as said above, you might want to keep them in a species specific tank.