Red Rhom is correct the general rule is 20-25gallons per fish.
however, (n many people have their own opinion on this)
I feel you can "bend" the rules a little when the tank size gets over 100gallons.
for example;
3rbp in a 75gallon
but
7rbp in a 150 gallon
tank size only doubled but piranha quantity doubled +1 with no harm.
(Idk if this makes any sense with how i worded it haha)
With that being said i would go as far as to say you could safely get away with 8rbp in a 160g if you're looking for a bigger shoal..but 6would be a good number with room for some tank mates if you can get them in...plecos n such.
and yes they're very hardy fish.. but verrrrry messy in everything they do so make sure you keep up on the w/c and have a powerhouse filter(s)