How many potatoes do you need to put on pie serving 4 people?

You don't put potatoes on a pie! Pie fillings are typically sweet, like fruit, or savory, like meat or vegetables.

If you are thinking about a potato dish that could be served with pie, you'd need to tell me more about what you're making. How many potatoes per person depends on the size of the potatoes, how they're being prepared (mashed, roasted, etc.), and the overall meal plan.