Unlimited. But, it all depends on your PC/server. Hardware with 4 cores, 8 GB of RAM without a GPU, you can pull up to 15-25 threads (depends on the configuration). On a PC with a GPU you can launch more threads. Just in case, the script has a built-in feature for skipping frames, forcing the browser to do fewer page renders, and you can boost the performance. But this can affect the quality of the working, so it’s better to take more powerful hardware.