How do I create a Banner with more than six layers?

This is a feature hardcoded into the game, so the only way to do it in survival would be to mod the banner layer limit. However, if you have access to commands, I believe this website could help you out http://www.needcoolshoes.com/banner . Basically, the way it works is that if you input the layer's design and color, it'll add to a command that gets generated when you choose the type that suits your needs. When you generate it, just copy and paste it in-game and you're all good!