2 head pillows for some people who are restless and would keep moving in the bed, so they don't miss any pillow zone with their heads. One for hugging. One for spooning them from behind for emotional support. One for crotch support to keep male reproduction organs comfortable by giving a bit of space between legs when sleeping on the side.
Remove one spooning or hugging pillow when having 2 people sleep in the same bed, then multiply the rest. You get 21, and that is the number of pillows you shall have.