well just the one red belly for now because of the size of the tank (25 gallon) which i know its too small the red belly is 2.5-3 in right now but i will be getting a larger tank as soon as the funds come along.
im sure youl get more professional answers from here. but i do think your fine for now, if you planning to upgrade soon. i think piranhas are pretty hardy, just dont over feed and your good