The "2.5 Slices" Rule
Professional caterers typically estimate that the average adult will eat 2.5 slices of pizza, and the average child will eat 1.5 slices.
However, context matters. If there is other food (salad, breadsticks, wings), people eat less. If it is a "Pizza Only" event or involves alcohol, people typically eat more.
Standard Pizza Sizes
This calculator assumes a standard Large (14-inch) Pizza, which is almost universally cut into 8 slices. If you are ordering Mediums (usually 6 slices), you should increase your order by about 25%.