A 10" cake serves how many people? not a wedding but for a normal family gathering, round or square...They like cake!

A 10"x2" round serves 15-20
A 10"x4" serves 39 (1x2x4 slices)

Depending on how much your family "really likes cake" I would even say you might assume 25-30 servings from a 10x4" round.

so really a 10x4 cake should be plenty for about 25 to 30 people and then if I add a smaller cake on top.