16+ cores

IMO: Basically, the first thing that would utilize all the cores is the software. If it is not optimized for X number of cores then the resulting experience is of usually a single or a dual core.
Most apps use 4 cores these day to the max.
Secondly, as the number of cores increase, the amount of current is drawn in parallel to make the CPU more energy efficient, thus, they usually have to be clocked lower than processors with lesser cores making them less faster at single core operations.

actually diminishing returns on performance doesn't really kick in until 64 cores. Its all about software efficiencies. The algorithms needed for most calculations loses efficiency because at least some of the program can't be parallelize, thus there is always something that you can't do with more cores.

You will always be able to do more than 1 thing with more cores tho.

Best answer selected by yumri.