Newbie with piranhas

Hi everyone,

I'm currently in the process of cycling my 55g tank, and hopefully soon, will be adding some piranhas in. I've received some conflicting opinions on how many to get. I know the general rule is 1" of fish per gallon, but some people have told me that 4 would be crowding my tank while others have said all the way up to 8. I plan to buy them as babies or pretty young so I can watch them grow and stuff but I'm more concerned about once they reach their adult size. Also, I've heard to always buy them in even numbers. Is that true? What do you guys suggest? Also, any other tips/recommendations in regards to piranha care? I'm new to the whole fish hobby so any info would be very much appreciated.

Thanks!!