The amount of cake required will depend upon the number of guests you have, how the cakes themselves are cut and whether or not you want to have some cake left over to enjoy at a later date. As a general rule, most wedding cakes tend to be cut into finger sized portions (ie 1” x 1”) but if you are uncertain, I would suggest asking your wedding venue or caterer how the cake will be cut.
The below guide should give you a rough estimate of what your cake will provide based on finger portion sizes:
45 guests- 7”/5” cake
70 guests – 8”/6”/4” cake
95 guests – 9”/7”/5” cake
120 guests – 10”/8”/6” cake
You will also need to take into consideration any guests who have food allergies, such as nut, gluten or dairy. You need to be extremely careful with a tiered cake as if any part of one tier comes into contact with another, there is a risk of cross contamination. I would, therefore, recommend that if you only have a small number of guests with a food allergy, boxed cupcakes or even a cupcake bouquet can be made so that your guests don’t feel left out.