what design are you going for? Petal on the bottom and then rounds? Alternating? The reason I ask is because that may effect how much space you want between the layers, to show off the different shapes. To be honest, the only time I've seen the two mixed is when there's a petal on the bottom and then just round on top. That being said, I would plan for 100, and say...
15" petal, 10" round and 6" round...serves 102 not including the top tier.
In that case, take out your pans and play around. That way you kind of know what you're going to have to work with. And you can use wilton's serving sizes as a guideline to see how many tiers you want to do.
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