I use 1 box for a quarter and 2 boxes for a half sheet cake. I bake the half about 35 minutes on 350 degrees and it works for me. 
1/4s and 1/2 sheets are different sizes to different people.
If you match the cake to the board & box sold as that size you will need:
1 cake mix for a 12x8; approx. 20 minutes at 325 degrees
2 mixes for a 11x15 ; approx. 30-40 minutes @ 325
2 1/2-3 for a 12x18; approx. 45-60 minutes @ 325
Each of those sizes is for a cake/pan 2" deep.
Time # are just a guide. You should bake them until you smell them, then test w/a toothpick or touch lightly w/you finger near the center; if it springs back any it is done. A well baked cake should not pull away from the edges of the pan. You should take it out before that
Everything you ever wanted to know about baking, assembling and decorating your 1st tiered/stacked/layer or in your case sheet cakes:
http://www.cakecentral.com/cake-decorating-ftopict-605188-.html
Has everything from cake and frosting/fondant recipes to baking help and so much more.
FYI, there are links to Wilton's cake preparation charts which give batter requirements by pan size.
If you're only doing a single layer, you only need to bake once. If you're doing a layered sheet cake you'd need twice the amount of batter (have to fill two pans).
The WASC cake recipe made w/two DH white cake mixes yields a tad over 14 cups of batter which makes it perfect for larger tiers or sheet cakes. (Recipe is in above thread.)
HTH
I use 1 and 1/3 box per quarter sheet cake, so increase the wet ingredients by an additional 1/3 of what is required, also it takes 4 eggs.
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