Use the Latin per capita when you want to say "per person" in an official-sounding way. For example, your town might have the most dogs per capita in your entire state.
English has more homegrown ways of saying "for each person," but this Latinate way of doing it (per capita literally means "by the head") now has a permanent place when you want to throw statistics around. It's been used in this way since the 17th century and shows no signs of slowing down.