Cuda has a texture limit.
Currently you can have up to 64 image textures, 32 float textures, and 4 HDRI RGBA textures and 4 HDRI greyscale textures.
You can use those textures as many times as you want in your materials, says you could use a texture 20 times but it is only one time in card memory.
http://en.wikipedia.org/wiki/CUDA
I think LightUp uses OpenGL and iirc they have a texture limit but this is dependent om VRam memory and texture size.
We hope all this limit goes away with later cuda versions.
My english is not so well, so i hope you understand.
Cheers mib