idk how, i have the same setup and 4 is just my maximum (and still epic fps drops )I have the same CPU as you, I can easily get up and run 5 VMs without any problem. On the 6 VM the load will be 100% on CPU.
Besides the CPU, I have 24GB RAM, 120GB SSD and a GTX 750Ti.
I recommend to change the stock cooler.
you use windows 7? Did you using some solutions to make vm\poe\eb work better?