As many as you like. But you'll need to have a hefty processor and plenty of RAM if you want to run them all at the same time.
Personally I would use VirtualBox. It's a far better product than VirtualPC.
if i choose to go with microsoft vm, can i install them on windows 7 64 bit desktop?
i am thinking of running at least four virtual machines on win 7 main desktop 64 bit , going by this plan what is your idea about the optimum processor to be used and how many GBs of RAM would you recommend if i am running all of them at the same time.
please note that since i am using 64 bit Windows 7 , i can have maximum of 24 GB of RAM installed on my motherboard.