As a cake artist, nothing irks me more than being told that my cake won't serve all the people they want, or that they payed way to much for a cake so small.
I then realized that most people don't understand how to cut a cake the proper way, ensuring enough cake for everyone, even if the cake looks "small."
There is a method to servings and pricing, so trust your baker when they tell you how many people a cake can serve.
Round, square, heart, and hexagon cakes have guides for cutting. Cutting a cake isn't rocket science, but since decorators usually price cake by the serving, it's considerate to your guests to look up how to cut a custom cake.
The thing you need to realize is that everyone uses different servings guides. There's a few basic guides, like Wilton's and Earlene's, but there are a lot of bakers that come up with their own cutting guides. If you ask, most decorators will provide you with a chart or guide for slicing up your cake, or they will at least tell you how to cut it upon delivery or pick-up.
If it's a wedding cake and you have a rented venue that serves food, they will usually cut the cake for you. They may go off of Wilton's wedding cutting guide, which many people find to be stingy. They may even slice pieces thinner than that, so there will be plenty of leftover.
(On a side note: beware of venues' servers stealing part of your cake. They may even go as far as stealing an entire tier, just to eat it themselves. I don't have first-hand experience of this, but I'd suggest sending someone to watch the slicing of the cake, like a bridesmaid.)
Here's an example of how a certain bakery may slice and price their round cakes:
You may notice that there is two different ways of cutting: wedding and party slices. Note that not every bakery will differentiate between party and wedding cakes when it comes to pricing a cake because the same amount of work will go into each.
Bakeries and at-home bakers usually price per serving, which can vary from $2 per serving to $15 per serving, depending on the area, what decorations you want, the different flavors of cake and frosting, and the individual baker's experience and skill level.
Don't be too generous when serving cake; most people don't want a huge piece, anyway.
Oh, and when ordering your cake, be sure to remember that you WILL be slicing into it. Be mindful of what you order.