i was wondering how many servings are recommended for a cake size of 12 x 18 and a cake size of 11 x 15

I think a 11x15 single layer is around 40 per my grocery stores, and i have no ideal on the other.

For rectangle cakes, it's easy to do the math to figure the servings.

11x15, when cut in industry standard 2x2x2 pieces, would be cut in approx 5 rows by 7 columns = 35 pieces.

12x18, when cut in industry standard 2x2x2 pieces, would be cut in approx 6 rows by 9 columns = 54 pieces.

12x18 if cut in larger 3x2x2 pieces, would be cut in approx 6 rows by 6 columns = 36 servings.

So if you know what size you want to cut the pieces, you can figure the number of servings.