I've had several brides come to me with this same scenario. On a regular wedding where cake is the only dessert, I usually tell them to plan cake servings on the 75% of the number of guests they think will be attending, letting them know if there is going to be alcohol served, it will usually be a bit less since guests that consume alcohol typically want salty snacks and not sweets, so they won't eat as much cake. When they tell me they are offering other desserts, I tell them to be safe, they should order AT LEAST for 65% of attending guests, since it is better to have too much than too little. Of course, the bride always has the final say in how big of a cake she wants, but I have them give me a count in writing. This has always worked great for me!