Use the Servings Panel to know how many servings your Design will cater to. It will show you the total servings based on all Cake/3D Cake layers that exist on the canvas. 

After adding Cakes or 3D Cakes to the canvas, your total servings for each portion size will be displayed in a list. It will also show you, based on the total price of the Design, the price per serving for each serving size set up in your Portion Settings.