Here's a cake tip for brides on a budget: Not everyone eats the cake.
I might get in trouble from other bakers for saying this, but I've said it before and it's true. Not everyone eats the cake, so you don't need to buy one serving for everyone. If you use 80% of the number of guests to estimate how much cake you'll need, you're going to have enough cake and you won't have much left over. If you buy one serving for each guest you're going to have a lot of cake left over.
That would be because the person cutting the cake cuts the pieces too small, or because not everyone has the cake, no matter how delicious it is. Somebody's always on a weird diet, people are too full from dinner, there's always someone who'd rather hang out at the bar and drink, and there are always people who are dancing or are just too busy talking to get cake.
For other bakers who don't like the thought of something cutting into their profits, let me ask you this: Would you rather have people say "Oh, there was so much cake left, it must not have been very good." Or would you rather hear "The guests ate every bit of cake, it was the best wedding cake I've ever had!"