What's the system you're running it on?
If I'm running solely Windows XP, I can run probably 10 of them easy with the rMBP and 16GB of ram. If I'm running more advanced OS, then not as much.
So far, I ran a max of 6 VMs at once on the rMBP listed in my sig. I ran it with 4 VMs inside ESXi that was running in a VM. So, VM inside a VM with a separate VM for the vCenter control in Windows 7.