How many adult piranhas can live in a 55 gallon tank?

One or two, and excuses my language, **** the other answers. I own two red bellies and i actually just bought a 55 for them yesterday. If you have any other questions forget this website and go to piranha-fury.com. They have everything you need to know and if not you can ask. Anyways a 55 might be a little snug for two but they take a while to grow.