How to add more than 4 cores to Virtual Machine in Hyper-V under Windows Server 2008 R2.
Go to XML configuration file of selected Virtual Machine and edit:
<processors> <count type=”integer”>Numbers_Of_Cores</count>
Where Numbers_Of_Cores is quantity of cores assigned to VM.
Official information about Requirements and Limits for Virtual Machines and Hyper-V in Windows Server 2008 R2: