I wanted to get a 12 core CPU (Threadripper 1920X) and I wanted to know the max amount of cores Blender can use?
Theoretically, Blender can use an infinite amount of cores, there are not set limit to this. Not as far as I know of though.
But cores don't really impact the performance on Blender renders directly, but it's rather the amount of threads your CPU can pump out and still maintain optimal speeds that matters. If you go to Cycles render, you've probably noticed the orange squares that travel across your screen eventually. Each one of these squares are threads, so having more threads will result in more squares. Bumping up the amount of threads manually isn't going to do you any good without a good multi-core CPU, like the one you mentioned.
So a short answer to this question; theoretically, there is no limit. So go ahead and buy a 100 core CPU if you so wish. Blender should (theoretically) be able to use all the cores.
Multi-core CPUs are almost always better at doing complex calculations at least the standard CPUs are (there are theoretical quantum computers which run with fewer cores, but at fenomenal speeds, but I won't go into this field).
I don't recommend getting yourself an expensive CPU like the 1920X unless you are professional in the field. Most software available for you today use up to 4 cores, but a majority still only fully utilizes two cores. Before I went into the IT business, I thought getting the AMD FX 8150 Black Box 8 Core CPU would boost my overall computing performance and speed up software such as Blender, games and such. It did speed up the Blender rendering times indeed, by a noticeable bit, but since most games out today don't use more than 4 cores, my performance actually dropped in some games.
So make sure you know exactly what you're going to use your computer for before purchasing one of these CPUs, as their designed use is almost exclusively for digital media editing/creation.
Just to add to FreemoX answer, I agree there should be no artificially imposed limit, as far as I understand, but there is also the obvious answer that in practice no one ever tried a 100 core processor, probably no developers even have access to 12 core processors anyway, so things are at best untested, possibly under optimized.
While it should theoretically work, in practice there may be limitations, bugs, unoptimized code that prevents software from taking full advantage of a very high number of cores.
Parellelization can also be taxing, there is also probably turning point below which it becomes inefficient to use more threads to process certain job. Splitting up the task, distributing workload, managing threads, becomes more taxing than processing it with fewer workers.
I know this question is a bit dated, but I wanted to chime in and say Blender 2.83 certainly seems to like 128 cores/256 threads on a dual 7702 EPYC system, configured as one singular NUMA node (0). Render times are phenomenal, and it's faster than a 64 core 3990x Threadripper, which runs at a much higher base and turbo clock speed, so it's using the extra cores. Kudos to the developers.