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.
It rather depends upon which OSs you are using. With 4 machines running you would really be best off with a quad core processor (or better) but if they are not very busy you might get away with a dual-core. The RAM calculation is easy, just add up the amount of RAM you would want if each of the VMs were a real computer and that's how much you need for the VMs - and then you need on top of that whatever the host OS requires.
So if each of the VMs was running Windows 7 you'd need at least 10GB, allowing a minimum of 2GB for each installation plus another 2 for the host. (Don't forget that if you are running Microsoft guests you need a licence for each VM.) XP or Linux guests are much easier of memory - say 1GB minimum per VM.
I'm afraid that that is not the case (as far as I am aware). The VMs run as totally independent machines so don't share resources with the host OS or with each other (except that you can choose to make shared folders available between host and VM).
The VMs run as totally independent machines so don't share resources with the host OS or with each other
so in that case, once i allot say 2 GB of RAM to each of four VMs, and my total RAM is 12 GB, does that means that my main host machine with windows 7 64 bit will be left with access to only the remaining 4 GB of RAM ?