Depends on how large a slice you want. I've done this and gotten only 72 1"x 3" x 4"deep slice. Or just do 2 single sheets then they can cut in squares 54 in each=108
Could you do two sheet cakes and join them together? If they are trimmed perfectly and crumb coated evenly I would think that would work. Or if you stack your 12 x 18 that should do 100 I would have thought.
There are a couple of discussions on Cake Central about this very subject, perhaps they will help -