The clam wings my father purchased for his build (the only part he did buy) were about 2 - 4mm thick, being 4mm or so at the edges but rapidly narrowing to 2mm (ish) for the main panel bulk.
As to how many layers you build up of what, The panel will be MUCH stronger if you build up of a higher number of thinner layers than by just using 3 layers to make your required thickness. I tend to use a layer of 30 to 80gm plain weave cloth for the layer immediately under the gel coat, and like to total between 3 and 5 layers as average for general lay up, but can go to 7 layers or more. Of course it'll vary depending on complexity of shape as well. I find I can also build much lighter this way, and coupled to a vacuum bag you can reduce the weight of a panel by half or even more.
Something to bear in mind - if the panel allows it, using a hard foam insert between layers will allow you to build lighter and thinner. (I get mine from the local boat builders yard)
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