I am fairly new at this but can you please tell me if there is a general rule for servings per cake. My friend wants me to make her a cake that looks like a vase (wider at the top) that will serve 70 people! I was going to make serveral cakes and stack them then shape them but am at the moment stumped, to say the least. I told her I'm a beginner but she still wants me to give it a go. She gave me a photo of the cake she wants, boy I must be crazy to even consider it. Can any one help?

Here's the industry standard chart:
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm