I am remodeling my shower and when I pulled the tile down I took down one layer of green board with it. There was a second layer under that attached to the studs. While doing research I noticed that some pics have 1 layer of green board and some have 2. Is there any code that guides the number of layers or is 2 just generally better? Can I get by with just one? Should I replace the green board with cement board?

Thanks, Jeff