How many cups does it take for an 18x12 cake. Wilton only has amounts for square. Please help.

A 12x18x2 takes 14 cups of batter.

Actually, the 12x18x2 is listed under sheet cakes in the 2" pan under party cakes:

http://www.wilton.com/cake/cakeprep/baking/times/index.cfm

The WASC cake recipe makes just a tad over 14 cups.

Here's the expanded flavors version:

http://tinyurl.com/2cu8s4

HTH