Windows server license

Some general info

Each host needs to be licensed.
A minimum of 8 core licenses is required for each physical processor and a minimum of 16 core licenses is required for each server.
Core licenses are sold in packs of two.
Standard Edition provides rights for up to 2 Operating System Environments or Windows Servers containers with Hyper-V isolation 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.
DataCenter Edition provides rights for unlimited Operating System Environments or Windows Servers containers with Hyper-V isolation when all physical cores in the server are licensed.

credit to: Dave Patrick 

Question on Number of CPU and Core for MS Licensing - Microsoft Q&A

in your case if your server is 16 core and licensed with Standard, then you can have 2 VMs without buying any additional licenses because standard edition will give up to 2 VMs within the license of your physical 16 core.