Find how many pizzas to order.
Pizza is ordered in slices and bought in pizzas, so the whole calculation is a rounding problem. Three slices is the standard adult allowance and two for a child, which for fifteen adults and six children is 57 slices — eight large pizzas at eight slices each, with seven slices spare. The variety split is worth calculating because ordering four kinds across eight pizzas means two of each, whereas five kinds leaves an awkward split and usually means one unpopular pizza going home. Keep it to a number that divides your order, weight it toward plain cheese and pepperoni, and expect the vegetarian option to need one dedicated pizza rather than a share of one.
Pizza Party
Slices needed = adults x slices per adult + children x slices per child; pizzas = slices / slices per pizza, rounded up; leftovers = pizzas x slices per pizza - slices needed
About eight large pizzas at three slices per adult. The quick rule is three eighths of a pizza per adult, so multiply your guest count by 0.375 and round up.
Three or four, chosen to divide your total evenly. Weight toward cheese and pepperoni, and make any vegetarian or dietary option a whole pizza rather than a share.