I could have gotten away with just one but I went with the two in case I decide to add another scene that I am considering that might be more than 2 inches deep. Still not sure on it yet. It also paid off at the bridge over the creek. The creek itself is just 2 inches deep but the abutments for the bridge were taller than that. I can tell you it was a lot easier to just trench a little foam to install them than trying to cut down those resin castings. It also didn't hurt that I was gifted the foam. If I'd had to buy it, it almost certainly would have been just one sheet.