Different organizations leverage virtualization in different ways, but virtualization is popular largely because it makes more efficient use of physical resources. In some cases organizations eliminate separate physical hardware for low demand servers, in some cases organizations high demand servers onto redundant hardware which allows load to be spread across hardware more efficiently while also providing failover capability, and in some cases organizations utilize the latest in virtualization technologies such as VDI (Virtual Desktop Infrastructure) to offload applications and workstation environments to the data center. Each case fits different organizations better, but the overall theme is to reduce unused resources and increase efficiency and uptime.
To answer your other questions:
Most hypervisors or virtualization solutions support an unlimited number of virtual environments. The number of environments that can be run is limited by the resources of the host and the level of responsiveness. As for the number of virtual environments a specific hardware configuration is able to support, this would depend largely upon the environments you intended to run. The hardware you mention would easily run hundreds of basic command line server environments and should be able to support dozens of graphic environments, but ultimately there are far more elements to performance of virtual environments than processor and memory alone, storage bandwidth and network bandwidth will have a significant impact, as will the level of graphics demanded by the clients.
You can read up about the virtualization offerings for the Windows Client in the Desktop Virtualization Center from The Springboard Series on TechNet including upcoming enhancements to desktop virtualization. You can also check out Hyper-V, the virtualization technology included in Windows Server 2008 R2 at the product page on Microsoft.com.