I would like to know single layer and double layer serving numbers for a 12 by 18 and also a 11 by 15. Also, is a full sheet classified as two 12 by 18's?

A half sheet is usually 12 x 18 (some vary an inch or so) Depending on how deep the layers or if it is torted or stacked, a normal serving is a 2 x 2 piece. that would make servings approx. 54. Wedding servings could be 1 x 2.

This was also discussed in previous threads:

http://www.cakecentral.com/cake-decorating-ftopict-53695-.html

HTH