You can do up to 8 textures on a terrain. D3D will not allow you to use more per material.(however, these can be large, and theoretically you could break each texture into a bunch of smaller ones and tile them seperately. You might need a DLL for this however..) But to tile 8 color textures you would need to modify the terrain multitexture shader from the WIKI. The key is to use each texture's alpha channel to blend it, instead of wasting a texture for the blender texture.