How many juvenile red belly piranhas can go in a 40 gallon tank?

The tank is about 4 feet long. How many little piranhas?

How many piranhas could i put in a 20 gallon tank?

I have a 20 gallon tank and would like to know how many piranhas i could have in it? The piranhas would be 1-2 inches at first and money is no factor. I just don’t know what would be best for the piranhas.