My sister had her wedding about 4 years ago and she had 300 people just for dinner and over 400 people for the dance. Her cake that she got only fed 150 people and they had a lot left over. To tell you the truth girls not everyone is going to eat cake. It all depends how you serve it. Some places will give everyone a piece even if they dont want it, some will ask everyone, and some will cut the cake and put on it on small plates by the cake therefore if you want it you can grab it on the way passed.
What are you doing for your serving size and how many people are invited?