For Windows Server Essentials(server system itself), it only allows 2 simultaneously sessions/connections at same time. Also, locally logged on user occupy one of 2 sessions. It is the license limitation for Windows Server Essentials edition.
Check the link (Concurrent RDP Administrator connections on Domain controller - Windows 2019 Server Essential) :
https://social.technet.microsoft.com/Forums/windowsserver/en-US/edf12886-d1a7-4c0b-806b-b84c5e1cc616/concurrent-rdp-administrator-connections-on-domain-controller-windows-2019-server-essential?forum=ws2019