In this case, I have 10 layers (one for each process stage). If I had to use them on a map, I would probably end up with 6 (1 merged outlines layer - step 1,2 and 3, 1 shadow1 layer, 1 shadow2 layer, 1 highlight1 layer, 1 highlight2 layer, 1colors layer), this will allow flexibility to play with the depth of shadows and highlights if needed. As for layers blend, you can either use normal mode with a high range of contrasted colors or use multiply for shadows and screen for highlights.