License are per core basis. The license covers up to 16 physical cores (8 packs of two cores). If you have more than 16 physical cores, you need to buy additional license packs. A license pack covers up to two physical cores. If you plan to deploy Nano Server, you need Software Assurance.
Licensing details: http://download.microsoft.com/download/A/2/F/A2F54CF2-FD52-4722-8C9A-728617C2AF17/Hybrid_Cloud_Windows_Server_2016_Licensing_Datasheet_EN-GB.pdf