Are you referring to Red Bellied Piranha? If so I would suggest only one for a 42 gallon, 55 gallons for two. They are cool to feed, but they are not really that aggressive, it can be hard to see them n the act of tearing apart fish or what not.
Don't Red Bellies get like 1-2 feet? I think a 55 is a little small for something like that?