They only require a license for VM's that run Windows. You can run as many non-windows VM's as you want on the free hyper-v server on as many procs as it will run on.
Oh, I didn't mean to imply otherwise. But you generally won't have to pay for a licence to run Linux or *bsd in a vm. I'd assume one would want/need to run Windows software on windows (reactos/wine aside).
