Do you have any VM's that use more than N-1 cores (real cores, not virtual ones)?
Sorry for the delayed response, but I thought I had responded to this question. Looking now, I don't see my response. Too many work and personal activities all running in parallel, I guess.
The answer to your question is both no and yes:
No: There is no single guest VM that has more than two cores. The only guest VMs that have two cores assigned are the newer OS X VMs, which require 2 cores. Most of my guest VMs have been assigned only a single core.
Yes: The combined TOTAL number of cores used by running guest VMs can exceed your N-1 = 3 criteria, which is 3 in my case. Sometimes the total exceeds N = 4.
The Mid-2011 iMac has an Intel Core i7 (Sandy Bridge) with four real cores (and four virtual cores - hyperthreading).
Can a guest VM lock out unrelated Mac processes and other VMs?
Is that the explanation - even though CPU utilization is low, guest VMs can lock a real processor core for long periods?