If you are starting cake decorating, you probably are a bit confused about how much batter to add to a pan or how much buttercream to make. Below you will find links to charts that will help you with cake servings and batter amounts depending on the cake pan you use. These are just guides to help you know how much you will need to make, so you are prepared ahead of time and not to waste time trying to guess what you might need. Eventually you will feel more comfortable about the sizes of the cakes and the serving amount each pan will give you. 
I hope these guidelines make you feel more comfortable making cakes as you start.