as much as you like... well up to 2,147,483,648.
Comes down to what are you processing, other hardware you are using etc.
Converting images, go for 100 and see how it goes.
Converting music, maybe 10.
Converting videos, without a GPU 1 or 2. With a GPU, is it unlocked, or does it max out at 3? What are you using for your temporary storage? HDD, SSD, NVMe gen 1, 2, 3, 4?
So in short, there's no magic number. Experiment and see what's best for your use case. Personally, I use 3, using a gen 3 nvme, a gtx 1650, and I mostly process videos. I do audio normalization which is CPU taxing though.