Assetto Corsa uses 10 textures, but you can use as many as you want. The game samples from a group of resources, the larger your resource pool the more diverse your sky. Plus, no performance hit :)
Thank you blk, still looking for more...even with 40 textures, once you have skyboxing that spawns over 80 clouds, the game begins doubling an tripling samples which really kills immersion...I'd like to get at least 50 or 60 eventually, but for now these are all I could find and modify :)