This is my first cake order of this size for a sheet cake. I am new and have just been making birhtday...etc. cakes in a 9x13 pan, I just purchased a 12 x 18 x 2 wilton cake pan..What would you suggest... 2 of the 12 x 18 or more., and how many cake mixes ( yes I use boxed but doctored up) for the 12 x 18? Thank you for your help

THe 12x18 takes 3 undoctored mixes. When cut in 2x2x2 pieces, you can get 50-54 pieces, so two of them would be 100-108. If you think they might cut the pieces bigger, three of the 12x18's should cover you ok.

Bake 2 of the 12x18s; stack one on top of the other for a cake that is 4" tall. You will get 109 servings 1x2x4.
Other wise, do what Indy said but add a 9x13 also. Make it either as part of the 'full sheet' or keep it separate (like a kitchen sheet).
Be sure you charge enough! I would say about $350. You will need to use a full sheet board doubled, maybe even 3 of them glued together for that much weight.

Here's one I made that is similar in size:
http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=27206

Thank you both very much.. will make 2 12x18 and 1 9x13.... thanks again and will double the cake boards..

You will need three mixes for that size pan.....