Well to get a better processor then that you would need to change the motherboard.
i7-6950X would be the most cores (10) at the highest clock speed. If you need more cores, Intel offers several Xeon models, some with up to 22 cores if I recall correctly.
Depends on what you are truly after, and how much you have to spend. High core count Xeons are upwards of $4000 and aren't usually available outside the US for a few years.