When using Parallels’ Virtuozzo on a single physical server, silos are created (running instances that act as the host operating system). Because of this, Virtuozzo is unable to run more than one edition of Windows Server on the physical server. With some products, Microsoft will allow you to run different editions of Server 2008 on different Operating System Environments (OSEs), but in the case of Virtuozzo, you (obviously) cannot take advantage of this.
If you are running all instances through the host operating system, all instances are exposed to all physical processors in the server, and therefore must be licensed on a per processor basis. That’s a lot of licensing, baby.