As you probably have heard Windows Server 2016 has core-based licensing, meaning that every physical CPU core has to be licensed.
Windows Server 2016 also introduces some changes how virtual operating system instances are licensed. Especially you have to take into account this rule:
Standard Edition provides rights for up to 2 Operating System Environments or Hyper-V containers when all physical cores in the server are licensed. For each additional 1 or 2 VMs, all the physical cores in the server must be licensed again.
If you have a 20 CPU core server, this means that if you purchase Standard edition licenses for 20 cores, you are eligible for 2 OSes, if you need two more OSes on the same physical server you will need to license all 20 cores once again.