The slider maxes out at the number of threads on your machine. There was a previous thread (no pun intended) in which Poser wasn't showing all available cores/threads on a dual xeon machine but I can't remember what the limit appeared to be.

With no technical knowledge of this other than a you tube review of the 3990x I believe Windows allocates work in 64 thread bundles and so won't use more than this for a single program unless the code is specifically written to take account of it.

I think it was this video

https://www.youtube.com/watch?v=1LaKH5etJoE&t=377s