There is no absolute maximum number of layers that you can add to an image request.
However, every extra layer increases the complexity of the request. At extreme levels, they can impact the response time. While the Image Servers can accept any number of layers, many browsers limit the number of characters submitted with a standard GET request. (This limitation is the equivalent of entering a URL in your address bar and hitting Go.)
If you expect to be working with large URLs (2048 characters or more), then consider switching to the POST method in your implementation. It does not have the same limit. The browser imposes this limit. (InternetExplorer allows maximum 2048 characters in a Get URL.) The next limit you run into is the one that the CDN defines, which is 4000 characters.