Sally Long has done some sleuthing on portion sizes for round cakes as follows:
Cake Pan Number of Servings
6-inch round 5-11 slices
8-inch round 12-16 slices
9-inch round 12-16 slices
9 x 13 sheet pan 15 large slices or 20 to 40 smaller servings
8 or 9-inch square 9-16 servings
Angel food pan 12-16 slices
Bundt cake pan 12-16 slices#
Hopefully this will assist those of you (inc me) who find cutting slices challenging.