Platies are generally pretty mellow fish, and do not kill each other. Just the males chasing the females when the females do not want to be chased.
2 males may get a little pushy with each other, especially with females in the tank, but they do not kill each other. These are not Cichlids!
The best ratio seems to be more females than males, and more than one male.
That way the males will spend a little time pushing each other around (ignoring the females) and some time chasing the females around.
As long as there are safe places for the females to hide, and more females for the males to chase, they will not chase any one female to death.
Stocking levels can get out of hand with these guys. That is what happened with my tank. Started with just a few, and they had more and then the babies had more... it sure did not take long to have a tank full.