1X, 2X and 4X refer to the amount of upscaling not simultaneous threads.
That said, I think their limits are a bit low. I can run 3 jobs at once quite easily on my system, but their algorithm is limiting me to 2. I pick up an extra 2 fps in the aggregate running 3 instead of 2 (which is another couple of frames faster than 1). Beyond 3 I get no gains, so I feel 3 is the right limit for my system.