Your approach is wrong. It should not be how many in a what.
There are many species of "piranha". There is no simple answer because they all have thier own requirements.
They are a shoaling fish, so you should always aim to keep 6 or more fish. Checkout the species you are interested in, and what is available.
Once you have an idea of what species to keep, ask again. Your current question is on the same level as "what fish can I put in a tank".