Install Windows Server 2008  Update for Windows Server 2008 x64 Edition (KB956710) enhance Hyper-V role's support for processors and virtual machines to now support up to 24 logical processors and 192 virtual machines. Read more at http://support.microsoft.com/kb/956710.

Note:

To start a Windows Server 2008 x86-based virtual machine on a computer that uses a 6-core processor, you also have to install hotfix 950182.
For more information, click the following article number to view the article in the Microsoft Knowledge Base:

950182 (http://support.microsoft.com/kb/950182/) A computer that is running an x86-based version of Windows Server 2008 or an x86-based version of Windows Vista may use fewer processors than expected if the number of cores on a socket is not a power of 2