Hello,
If you want to use RDS, you have to purchase CALs if you want to have more than 2 users logins at the same time.
You can have 2 users login to your server if you didn’t purchase any CALs under admin mode. (Note you should not install any rds role, otherwise you need to use "mstsc /admin" instead of "mstsc:)
About CAL, you can follow this document: https://learn.microsoft.com/en-us/windows-server/remote/remote-desktop-services/rds-client-access-license
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