There are 5 Peace Souls. The ones that got them are: Ryoma, Kogoro, Shinsaku, Soji and Kondo. But after Kondo died he gave his Peace Soul to Hijikata.
They are a piece of Ultra Soul. Ultra Soul is when all human hearts reach their greatest height. It's scattered fragments are Peace Souls. When Peace Souls come together the Ultra Soul is born. It's a powerful soul that dramatically changes the flow of an era.
You can obtain the Peace Soul when a shooting star falls on you, but you have to awaken the Peace Soul yourself.