The number of slices one can get from a cake made in a 9 x 13-inch cake pan depends on how big the slices are. This rectangle-shaped pan makes it easy to cut square-shaped slices. On average, there are 15-18 slices with this cake pan size.
If more than the 18-slice average is needed, one can always cut the pieces smaller. If it is going to be served with something, like ice cream, guests might not mind having smaller slices. One way to give the illusion that the cake slices are larger is to slice the cake in half horizontally, and then add a layer of frosting between the slices.