Virtual computer lab with Windows Server 2012

I am setting up an Active Directory Domain and have three computers to do this on, one is running the server another will natively run windows 7 professional and on my last pc, my gaming rig, I would like to have multiple instances on a professional grade of windows 7 or above. Relevant specs are : Windows 7 professional, 16GB Ram, 300 GB spare HD space, FX 8350 at stock. How many instances of windows do you think I could run at once? Also anyone know how I could get multiple free/cheap copies of Windows because it seems like a complete waste of money to buy them? Thanks.

Depents on what you are planning to do. If you just want to get farmiliair with Microsoft's AD. Or what to get in exchange, azure, adfs or something. Do you have more info? And if it is an test envoirment. Why not sell your 2 extra pc's and buy an SSD to put in your rig to put your vm's on.