I was asked to do my first cake. It is a 12x18 sheet cake which takes 3 cake mixes. If I add an extra egg to each mix will the cake overflow its borders and cook all over my oven? If yes, how many extra should I add? 
The lady wants Tweety and Mickey Mouse on it. What is the advantage of doing fbct over just doing it on the cake? I have never done a fbct so am not totally sure, but kind of have the idea of how to do it. 
Thanks in advance for you help.
Mary
I've made quite a few 12 x 18 cakes and used 3 mixes. I always use an exta egg and a pudding mix for each cake mix box and it won't overflow. I will rise above a little but it won't spill over into your oven.
I've also done two layers of 2 boxes each (definitely doesn't extend past the top of the pan) so I didn't have to torte a cake that big. But when you factor in filling, you end up with a pretty tall cake. Not sure if you want filling, etc.
I am not getting paid for this one, so don't want to put a lot of extra into it. She is buying the supplies.
The last time I did the egg thing was in the little oval pans that come in the kit for course 2 and I still need to clean my oven. 
That was a couple of weeks ago. Those pans were already close to over flowing before I added extra egg. I should have collared them. Just curious about the sheet cake.
thanks awolf24. No, we are not filling the cake. I did not offer. She did not ask. I will feel comfortable putting all them eggs in there. I do the pudding thing to. I also sub the milk for water and add 1 tsp of flavoring too the mix, sub single serve apple sauce for the oil and you have one tasty cake. 
Thank you every one for your quick replys to my questions. 
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