I was asked to make a number 50th birthday cake.  I found how to create the numbers from cake central, however my question is should the cake be a double layers or single?  This is new to me.  Thank you for your response.

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.