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.

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

why 9 cores ? why 7 SPU ? as far as I know a PS3 game can only use up to 6 SPU, because 7th is used in internal by the OS and 8th is disabled. So it sounds for me like 8 cores.

Yeah, maybe so. I just quickly looked at this. "the PS3 can have at most 9 threads runnings at the same time (2 from PPE and 7 from SPE)." But yes, doesn't mention what games can do.