You will need a Server License + Device CAL . Client Access Licenese or CAL allows any number of physical users to access Microsoft server software through a single device.
With most products, you must assign a license to a server for each instance of the server software that you run on that server. You may run that instance in a physical or virtual operating system environment on that server. By exception to the licensing model, some products provide broader use rights. For example, a Windows Server 2008 Enterprise license permits a running instance in up to 4 virtual operating system environments at a time on the licensed server. With some products, such as Windows Server 2008 Datacenter, you may run any number of instances at the same time as long as you license each physical processor on the licensed server.
For Windows Server 2012 you can refer to this link please:
http://www.microsoft.com/en-us/server-cloud/windows-server/buy.aspx