yes, a 55 is just about 12x48 (give or take half an inch)
it depends on how deep you want it. a standard 50lb sand is about half a cubic foot. which would be like filling a container that is 12x12x6. so if you want your sand to be 3" deep, you would need aproximately 2 bags, if you want it to be an inch and a half, you'd only need one bag, if you want it to be 4" deep, you may need a third bag, but you probably wouldn't use all of it.