Hi,
Windows Server Essentials has it limitation – supports up to 25 users/50 devices.
If your user or device number will exceed the limitation, it is recommended to migrate to Standard OS version.
Growing Beyond 25 Users with Windows Server 2012 Essentials (it is also applied to Windows Server 2016 Essentials):
https://techcommunity.microsoft.com/t5/windows-server-essentials-and/growing-beyond-25-users-with-windows-server-2012-essentials/ba-p/398384