skeelo (???) it depends on the species of piranha. The stereotypical red-bellies are shoaling fish in nature so should be kept in as large a group as possible, 6 being a good number, 4 being the absolute minnimum. They each grow to 12 inches as well and, if not well-fed and in a spacious tank, they will become canibalistic. The fish themselves can be quite expensive - not to mention the 250 gallon tank you would need to keep them comfortable!
However, there is, I believe, a solitary species that can be kept in a much smaller tank completely on its own. Finding and buying this fish could be expensive but the tank would be average. You'll need to do some more research to find out about this fish - I think it's called a 'black piranha' or something like that - can't remember.