Most video cards support only 8 multi-texture units. So logically, 4 alpha, 4 textures.
Could you maybe limit it by having an option? Unless you know how to achieve the 9 + 9 current textures that you use. And I definitely am not going to render the terrain 2 times because thats insane. I would have to then draw it 4 times to get water reflections at run-time.