As the general rule is at least 1 core per game instance then the new cpus from AMD and Intel both fit the bill for the majority of people 5 or 6 boxing.
The other recommendation is to not go below 3GHz for the cores. There were several people using high core count Xeons, clocked at 2.1GHz, and they found that they just weren't fast enough for a single game to play nicely, despite the 20/20 cores.
So, is more cores superior. Sure, but only when the GHz is not being compromised so much that it affects performance.
The new Ryzens and Coffee Lakes both meet both the recommendations in the mainstream desktop range, so you can keep your kidneys.
The other peeps on
http://dual-boxing.com who have Ryzens are very happy with them, and they turn out to be
not too bad at all.
There are also some
real world performance tests for multiboxing on an R7 1700.
All in all, the new competition from AMD and the now catchup from Intel is prompting affording CPU's into the range of more multiboxers, so it is much easier to get an appropriate hardware.