none. Right now they are all in one directory.
the only option you have is to use different TWG installation and link them together.
The maximum number I tested was 200.000 images but this was on my windows machine. I don't know how different systems behave.
In 1.7.5 I already refactored that the cache names are build in the same function. Therefore it would be possible to extend the function to create subfolders. I just did not do this because I would have to change my cache cleanup function as well and I did not have the time to do this.
If you don't care about the cleanup the cache with subfolders should be no problem.
This is on my todo list for 1.7.7
.
/Michael