OEM-, ROK- and FPP Licenses
Windows Server 2008 OEM-, ROK- and FPP Products are supplied with 2 license keys, one Product Key and a Virtual Key.
Importantly, you must use the Product Key for the Physical System (Parent Partition) and the Virtual Key for the Guests (Child Partitions).
If a Product Key is used in a Child Partition, then you may need to activate the Child Partition again if you move the VM to another physical server.
As to how many Child Partitions that can be activated with a Virtual Key depends on the Windows Server Edition and the definitive resource is http://support.microsoft.com/default.aspx/kb/949748
Volume License Keys do not differentiate between Parent and Child Partitions and only 1 key is supplied. It is recommended that a KMS Server is used to manage Volume Licenses which will re-activate the system automatically if needed. How to setup a KMS server is documented here: http://technet.microsoft.com/en-us/library/cc303280.aspx