at that size you can put a few in there but as they get bigger you'll eventually reduce the number. reds have potential to get to 12" so a 55g would be too narrow for them, even at 9-10" I would recommend to move them to a 75g if possible. if 55g is all you'll have available then personally i wouldn't keep more than 3 at adult size. some might get eaten along the way but if all live then you can sell some off.
the other thing I would like to point out is that I see you only have one HOB filter, with piranhas they are very messy eaters and produces a lot of waste so you'll eventually have to upgrade the filter sooner or later. Try to get a large canister filter if you can, Ehiem 2217 or Fluval FX5 are both great filters. You can use the canister filter as main bio filter and keep the HOB as mech filter.
how are you cycling your tank?