how many cores/threads rpcs3 uses?

At least as many as the games themselves use, so for high end games I guess 9. 2 PPU threads, 7 SPU threads. And since we are an HLE emulator some things may be able to run on their own thread as well. (If the game just tells the PS3 OS to do something, we can do that in a new thread, possibly, I am guessing this)

Further optimizations for more parallelism may or may not be possible in the future.