If you want a very thin cake, you could do just one layer, although it'll only be 2"-3" high, or however high you make that layer. I personally would do two layers of cake with a filling, frosting, or ganache in between.
Additionally, keep in mind how many servings the customer wants - if you do a single layer, you'll get fewer servings.
More importantly is how many servings your customer needs. I often made single layer cakes and don't think of them as 'very thin'. We are all different:)
Look through my album - I have tried to note sizes on most of the posted cakes....it could give you an idea of how it will look if you do the single layer.
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