People have managed to fudge in extra layers though with the use of shaders and such so you might be able to find some useful information. Certainly texture splatting attempts have been made and achieved extra layers, somehow.
The basic setup is for 4 texture layers per material (you can have arbitrary many materials per object). You can easily reduce that number, but adding some more needs additional treatments, because the texture matrices are not automatically adjusted to the layer number. This can lead to render state corruptions.