Oh boy, did a bride and I make a bad decisions once. I made a 7 tier cake for this awesome lady, and she still orders something every once in awhile. But she decided she wanted a different cake/filling for every tier. And then I thought of the brilliant (<----said sarcastically) idea to make a framed menu to put next to it. We were both excited. So there sits the cake, with a framed menu next to it, and I get my pics and go home.
Fast forward a year later when she comes to get her anniversary tier. Come to find out, there was no proper service that night, guests were left to their own devices when it came to the cake. So what happened? Yeah, most of them took several slices from different tiers.
Bride and groom never got a slice. She wasn't mad, in fact she was happy that they liked it so much that it all got devoured and got lots of compliments, but I use that incident as an example now when people start getting carried away with choosing flavors. I tell them it could be a fun thing to do, or it could make people take more than one slice, so if you choose to get many flavors, watch how it's served.
Good Lord, what a bunch of pigs! I tell people that when you have two flavors people tend to choose one and they're happy, but when you add a third there's something about it that makes people want to try it too. That's something else to figure in to how many servings are needed, but it still doesn't mean that you need 100% of the guest count. In my experience the biggest factors are age range of guests+open bar+other desserts/food