If this is for game, using one map for diffuse when it comes to props is the way to go. More maps means more drawcalls and more drawcalls is the very cock-cheese of satan. So only use more maps if the desired effect is not achievable with the maps you already have. It usually goes Diffuse, Normal, Specular and Gloss for…
That depends on the engine. JPG is an absolute mess internally, so it's not ideal for games at all. Web graphics use JPG just fine because some data loss is acceptable. Normal maps need to be saved in a loss less format because the pixels represent directions and not color. Typically you save all your texture maps in the…