At least 4 cores per virtual machine running a Lync server role - that are constantly available JUST for that box. This is to use the Realtime codec translation.
This topology includes a physical host server running one virtualized Lync Server 2010 Standard Edition server. You can also deploy an Edge Server (either physical or virtual), and Monitoring Server or Archiving Server (either physical or virtual). If you deploy either of these roles on a virtual server, that virtual server must be hosted on a physical server that is separate from the server hosting the Standard Edition server. You may also deploy a Director (physical only, not virtual).