With the release of each version of Windows Server, the maxima for supported number of sockets and logical processors have increased.
Whatever the numbers for the older OS releases, the best scalable platform for virtualization is Windows Server 2012 (and of course Hyper-V Server 2012), far exceeding VMware 5.1 and others.
Let’s start by defining common terminology.
Logical Processor: A thread of execution on a physical processing unit which can be a core or a thread on a symmetric multi-threaded (SMT) system.
Virtual Processor: A virtualized instance of a logical processor exposed to virtual machines.
Socket: What a physical processor plugs into.