You actually need to have the license on the paper. This allows you to install either 4 instances (Enterprise) or unlimited (Datacenter) Windows instances - on the same host - using the same key. Depending on which license (Retail, Open, ...) you purchase you also have a downgrade/downedition right. Btw. a Windows 2012 Standard license allows 2 virtual Windows instances and might be less expensive than a Windows 2008 Enterprise edition.
Anyway, whatever license you plan to purchase, make sure you read the Microsoft licensing white papers to get the right one that fits your needs!!
André