With regular Minecraft you're unable to have more than 2 layers on your skin. One being the base, and the other being for the accessories or whatever you want to use it for. I'm not sure if there's mods that could allow you to add more layers to your skin, but I do know some mods let you change your skin in-game and use pieces of armour to add things to your skin and make them look like another layer. For example, I know there's some superhero mods that use pieces of armour to make it look like another costume on top of your skin.
Apart from that, I don't think there's many more ways to put more layers onto your skin and if there was it'd probably only be visible to you and whoever else might have the same mod installed.
Long story short, if you wanted to; you could incooperate a texture pack into your skin and custom design pieces of armour to go with your skin.
