I have an order for this cake to feed 12-15 people. Will a 3 or 4 layer 6 inch round cake be enough?
The wilton chart says 6 inch round serves 12, earlenes chart says it serves 8. I think you could get 12 slices from a 6 inch round if you were told in advance you needed to get 12 slices.
15 servings is pushing it though. I'd tell them you can do a 6 inch and it will serve 12 (if slices are thin) or the next size up is 8 inches and they'll have to pay for that many servings.
If you do 4 layers, will you be putting a cake board in the middle, basically splitting the cake into two, 2-layer cakes? If you do this then a 6 inch round cake will be enough to feed 15 people since you basically have two 6" round cakes. But if you don't split the cake with a cake board then you will get 12 slices out of the 4 layer cake. So at that point you would need to up the round to 8" to give you additional servings.
Also, if this is for a party at someone's home, most people do not know how to cut a cake like a caterer, meaning they will most likely cut slices bigger then "normal." So a heads up on how to cut a cake to get the appropriate amount of servings that it was made for may be in order.