I think it is hit by the same limit as with all other xml files, meaning max 32768 entries (non negative values for a signed 2 byte int). You will run out of memory long before hitting this limit. That is unless most styles points to the same graphics, but then what is the point? I'm not aware of any limits other than this and memory limitations. I would pay attention to memory usage if you want to go all out on this though. No graphics are good enough to allow a mod to become unstable due to running out of memory.