The following chart explains how many slices you will get from each round cake tier and also is the basis for pricing (see pricing tab). 

6-inch pan serves 12 slices
8-inch pan serves 20 slices
10-inch pan serves 36 slices
12-inch pan serves 46 slices
14-inch pan serves 76 slices

and from each square cake tier.
6-inch pan serves 18 slices
8-inch pan serves 32 slices
10-inch pan serves 50 slices
12-inch pan serves 72 slices
14-inch pan serves 98 slices
16-inch pan serves 120 slices

This is a pretty good idea for wedding cakes, but not for birthday cakes. Usually you eat a larger piece of cake at birthday parties, so I recommend cutting the serving size number in half and that is a better number.  Pricing is according to this chart.