Hello,
Based on the information available, User Client Access Licenses (CALs) are required for users or devices that will directly or indirectly access the Windows Server. In your case, since the Windows Server 2019 is being used to install Microsoft Entra Connect for syncing AD accounts to Microsoft 365, and only administrators will have direct access to the server for managing Entra Connect, you may not need additional User CALs for regular users.
For a definitive answer, it's recommended to consult the licensing terms provided by Microsoft or contact a Microsoft licensing specialist, as licensing can be complex and there may be specific terms or conditions that apply to your situation.
Best regards,
Karlie