This is a question I get from every single bride and groom. There are a lot of differing opinions out there. For me, the answer is quite clear: it all depends on how you decide to serve your cake. I'll give you a few scenarios and what I would advise for each one:
"Our wedding cake is our main dessert. It will be cut and plated by our venue and served to each guest at their seat."
The answer is very clear here. You need at least as many servings as you expect guests. If you are expecting 75 guests, you need at least 75 servings. If you are expecting 250 guests, you need at least 250 servings. Easy peasy.
"Our wedding cake is part of a sweet table/buffet. Guests will have the option to take a slice of cake along with other desserts, fruit and delicacies."
Here is my somewhat controversial opinion: don't get more than 2/3 the number of servings as guests. In fact, if you are having an extensive sweet table, I would even go so far as to suggest you go with half. So, if you are expecting 200 guests, go with a 100-150 serving cake. I've been to a lot of weddings. Many of them with sweet tables where the wedding cake was the centerpiece. Sadly, most people pass on the wedding cake in favour of other goodies to fill their plates. I can also say, with complete confidence, that I have never seen more than 10-20 men, at any given wedding no matter the size, accept a slice of wedding cake. Assuming that about half your guests are men, you've just eliminated a good portion of your cake-choosing guests. I have never had a client come back to me to say that they didn't have enough cake. What I usually hear is: "We should have taken your advice. Luckily your cake was so delicious we didn't mind eating it for a week."
"We would like cupcakes/mini cakes, but they will be part of our sweet table."
While still part of a sweet table, my usual advice won't apply here. There are very few people in this world who don't like cupcakes. Seriously, nearly everyone likes a cupcake. Eating things with our hands is much more appealing than eating with a fork. Plan on ordering at least 2/3 the number of servings as guests, if not equal amounts.
"But what about....."
There are always exceptions. Only you know your guests and their preferences. Ultimately it is your decision. If it's a tradition in your family that no one can leave the dance floor until everyone has eaten their equal share of the cake, then order accordingly. If you have your heart set on a 7-tier couture cake, but you are only expecting 40 guests, you can have that too.
Deciding on the size of your wedding cake shouldn't be rocket science. Use your own good judgement (and a little of mine) and we'll find a good fit for you and your budget.
Have a happy cake!
Baking, Decorating and all things cake!