Layers are specific to the style option or the final look that one is going for. While there is no specific limitation to the number of layers that one can opt for, there is a basic rule book that is created based on the final look that you are going for!
So if you are going for a shorter style then you can go for 3 which is the basic number when it comes to this style statement. By shorter style we mean that the length of your hair is at least crossing the nape of your neck. However if you have thick curls or waves then you can go for the three layers option even with hair length that just reaches the nape because the style is still very much out there for you!
As you increase the length of your hair you ca increase the number of layers but do not go for a number higher than 5. Just increase the distance between the layer and make sure the top three are closer. This is a very cool style statement that can surely make a lot of difference even if you just leave your hair as is and not use any hair products except just your loyal comb!