AFAIK, that's depend on the Product-Key used. As we upgraded a Server 2008 from 100 to 250 User (legally), we received a new P-Key from MS.! Upfront, we could use to install and connect more that 100 user, just it wasn't possible to access the server with more than 100 users at the same time. Not upgraded to Server 2016 (yet), so I couldn't tell!
It is officially supported by Microsoft for Windows Server 2016 Essentials to do a License Conversion, so you can convert to the full retail version by entering a retail, volume license, or OEM key in the command slmgr.vbs.