Custom Canvas window.
Source: iMore
Here you will be able to select the dimensions in millimeters, centimeters, inches, or pixels. The dimensions and your hardware determines the number of layers that you ll have available in your project.
The smaller the dimensions and the more powerful of an iPad you have, the more layers you ll have at your disposal. Unless you re doing a very complex piece, you probably won t need hundreds of layers. However, if you plan on doing larger artwork with a lot of layers, you may want to consider getting an 12.9-inch iPad Pro since it ll give you a larger surface area and better hardware specifications.