If you're running the game INSIDE a virtual machine, then performance will be affected because of the virtualization of the display interface.
If you're running the game on the host OS itself, then performance should only be affected if the game is memory-limited or if the CPUs / disks are busy due to VM activity.
Thanks for the answer Sminlal!
I think the activity of the apps I'm running in the VM are not to demanding on the CPU. Each VM will be running a game that is not really demanding (magic online, it's a card trading game) and an application (MTGO library) waiting for ̎ customer ̎ to trade with it. So is it reasonable to think I might run 2 VM and play a game at the same time and how can I notice if it's too much?
By memory limited you mean not enough RAM?