O Essential é uma licença 1: 1. Pegue a mesma chave da instância física para ativar a instância virtual.
A licença OEM anexa essa licença, física e virtual, a esse servidor.
Essentials edition allows to run one instance of the server software in one physical operating system environment, and one instance in one virtual operating system environment on the licensed server. If you run both permitted instances at the same time, the instance of the server software running in the physical operating system environment may be used only to run hardware virtualization software or provide hardware virtualization services.
Para a compra de CALs, acho que o Windows Server 2016 Essential ainda segue seu método de licenciamento predecessor. Veja isso para referência;
Please note that although licensing for Windows Server 2016 Standard and Datacenter will follow the core-based licensing structure, Windows Server 2016 Essentials edition will continue to follow the processor-based licensing model used in previous iterations of Windows Server. Licensing continues to follow a per-server licensing, where one license covers one Server with up to two processors and Server CALs are not required.