This chart should have all your answers - I use it all the time!
It says a 1-layers 12x18" (2" deep) cake takes 14 cups of batter. A 2-layer 12x18" cake serves 72 so I would guess the 1-layer would serve around 36.
For the 11x15" (2" deep) cake takes 11 cups of batter. The 2-layer 11x15" serves 60 so I guess a 1-layer serves 30.
The site also has baking instructions - cook both at 325 degrees, the 12x18" one for 40-45 minutes and the 11x15" one for 35-40 minutes. I always have to keep an eye on them though - seems like the time is usually off (I have to leave them in longer it usually seems like).
Hope that helps. The chart on that site also shows how much icing you'd need!
at 10:19 AM on Feb. 20, 2009