The cores - probably the sky is the limit. Linux is born out of a server OS so multitasking und threading is not a problem. If there are some new instructions however - it is a matter of firmware and support in the kernel. Given that probably about 30% of the linux kernel source nowadays is contributed from BIG CORP (Alpha, Beta, Meta and co.) which need performance for their server farms to power the ASB (Artificial-Stupidity-Baloon) there will probably never be any problem with newer cpus in linux. Still, i would wait if it is some kind of embedded system on a chip, because sometimes there are delays in the drivers/firmware for some of the subsystems, like wifi. But for desktops i would not worry.
Nvidia…just do not buy it. They hate linux or at least do not care, and you will suffer. Just look at the number of support topics about their stupid drivers.