Giving you a “correct” answer is impossible in this case to be honest. There are so many reasons which determine how many machines you should be able to run parallel without issue that focusing on pure core count is prohibitive.
Things like the architecture used, SMT support, support for enhanced acceleration features, clock frequency, pipeline depth, instruction sets, the frequency at which your CPU runs (though like the core count, isn’t as important as it may seem), optimization and generally how well your host OS has been set-up all can have a massive impact on this.
This can go such a long way that, in certain scenarios, I am able to run multiple machines (1 Gateway and 3 Workstations) on a low voltage, low clockspeed and low TDP Skylake Dual Core CPU without many issues.
So, in conclusion, due to the complexity of processor technology, your question can not be answered that easily.
Have a nice day,