I would say 3 mixes per 12x18x2, if you're not extending or doctoring them.
Here's a handy chart (uses DH mixes):
http://forum.cakecentral.com/cake-decorating-ftopict-22705-.html
(ozzynjojo's post)
If you use the doctored White Almond Sour Cream cake recipe - 2 boxes would be enough:
http://forum.cakecentral.com/cake-decorating-ftopict-22705-.html
http://tinyurl.com/2cu8s4
(More flavor variations.)
HTH
If you look in the pan their are two lines, one for a single layer cake or a double layer.
Single layer - two cake mixes.
double layer - four cake mixes
Huh???? Isn't a "double layer" made up of two cakes with filling in between?
I just took one out of the oven. I used 2 mixes with an extender. It rose all the way to the top of a 12x18x2.
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