How do you know how much your cake will serve if you are sculpting it? Basic serving charts don't apply if you cut pieces away. What do you use?
An easy way to do it is to weigh your cakes. Say your 20 serving cake weighs 2lb. If you cut away the cake scraps and weigh them in a bowl and find you have 1lb in there, your cake now serves 10.
If your sculpted cake is an easy shape, you can also "guesstimate" how much you have to cut away. So if you only cut the equivalent of a couple of servings off your original 20 portions, you will have 18 left.
You should still charge for the 20 servings you started with though.