There’s more than a few questions about user limits, device limits and so on around Windows Server 2012 Essentials. Microsoft have a few blog posts, on this but are not really all that clear – so here’s my shot at answering those questions.
How many users can I run on my Windows Server 2012 Essentials server?
Windows Server 2012 Essentials comes with support out of the box for up to 25 users. You do not need to purchase additional CALS at that point – in fact there are no CALs that you can purchase for Windows Server 2012 Essentials at all. What you have when you purchase Windows Server 2012 Essentials is the right to create up to 25 users and allow them to access the server and it’s resources – note that I did not say that you have 25 CALs for users.
What if I want to have 26 or more users?
If you want more than 26 users, you can do that using the Transmog or inplace upgrade available from Microsoft. They describe it here. What you do here is basically get a Windows Server 2012 Standard Edition product key and enter that into your server – and after a couple of reboots, you have a Windows Server 2012 Standard Edition server with the Essentials bits still present and working. Two things you need to understand. Firstly – when you do this, you need to purchase Windows Server 2012 CALS for all users or devices that access this server – there is no limit to the number you can purchase – if you want you can put 500 users on your server now because it is basically a Windows Server 2012 Standard server now. The second thing you need to understand however is that the Windows Server 2012 Essentials features that were left on your server are only tested and supported for up to 75 users. So while you CAN have 500 users, Microsoft will tell you they only support up to 75 users as that is all they have tested and will guarantee. The article I linked to above will give you more information on that.
Boon Tee – fellow SBS MVP has a blog post here where he also summarises what Microsoft are trying to get across… should not be so hard should it? 
I’ve a windows 2016 essential, with 15 users connected.
on a separate lan (separated by firewall) i have several users connected only to web via wifi.
Can i use radius server ( windows NPS) to authenticate wifi users? (just to authenticate, not to log to 2016 domain)
Thanks
Hi Gabriele,
Whilst I’ve not yet tried it with 2016, the NPS role is installed already, so you should be able to do what you want. Let me know if it works.