So the rule of thumb is no more than N-1 virtual (vmware) cores per any individual VM, where N is the number of physical (not virtual, not hyperthreading cores in the machine), and ideally no more than N-2 (so there's always 2 for the host).
Hyperthreading is irrelevant in this case, it's physical cores - the details are somewhat arcane, but it has to do with how both the host and guest OS lock them during use. That causes resource contention and the machine starts thrashing, slowing overall performance (and in some cases crashing).
You can run multiple N-1 virtual machines at the same time. I have an 8-core i7 (8 real cores), and can run 2, 2-core, and 1 3-core VM's simultaneously. The machine's slow, but it works (as long as nothing is spiking any CPU usage). But there's a dramatic speed improvement across the board if I never exceed a total of 6 cores in use by VM's.
There are some special cases, especially on 2-core hosts. Windows 10 and OSX both require 2 cores to the guest in order to function properly, so there will always be resource contention. For those old machines, upgrading really is idea. On a 4-core host, I wouldn't run more than one 2-core guest at a time if there's any significant load on the host.
