Microsoft Lync Server 2010 communications software supports virtualization topologies that support all Lync Server 2010 workloads—instant messaging (IM) and presence, conferencing, and Enterprise Voice. Windows Server 2008 R2 is required, and both the Hyper-V and VMware virtualization platforms are supported.

The supported topologies are as follows:

  • Standard Edition server topology for proof-of-concept, pilot projects, and small businesses. This topology supports up to 2,000 users per virtual Standard Edition server.
  • Data center topology, for larger deployments. This topology supports up to 5,000 users per virtual Enterprise Edition Front End Server.

More details here:

http://technet.microsoft.com/en-us/library/gg399035.aspx