I always ask how the cake is going to be served. Many places in my city take the cake back to the kitchen and cut, plate it, then serve it to each guest. Then you need a piece for everyone. If the cake is cut and served on a sweet table, then guests come up and get their own piece. Not everyone will take a piece of cake.
For graduation open houses, so many people go to several open houses the same day, they usually don't eat cake at every house!