Hi,
If you have RDS roles installed on the remote sever, mstsc /admin will allow users to remotely administer the server.
But if there is no RDS roles installed, it is not mandatory to use the /admin switch. In this case, the same connection behavior occurs with or without the /admin switch. At any point in time, there can be two active remote administration sessions.
To start a remote administration session, you must be a member of the Administrators group on the server to which you are connecting.
Reference link:
https://support.microsoft.com/en-us/help/947723/changes-to-remote-administration-in-windows-server-2008
Thanks,
Jenny