Well, as many as they want. Some people may be hungrier than others. If you want them each to have the same amount of slices:
5 pizzas8 people=58 pizzas per person
5
pizzas
8
people
=
5
8
pizzas per person
Cut each pizza into 8
8
slices:
408 slices per person=5 slices per person
40
8
slices per person
=
5
slices per person