To determine the licensing requirements for a new Windows Server Domain Controller (DC) in an existing infrastructure, you will need to consider the number of users that will be accessing the server and the version of Windows Server that you plan to use.
- Windows Server CALs: A Windows Server CAL (Client Access License) is required for each user or device that accesses the server. If you have 120 users that will be accessing the DC, you will need 120 Windows Server CALs.
- Windows Server version: The version of Windows Server that you plan to use will determine the type of CALs that are required. If your existing servers are running Windows Server 2012 or 2016, you will need to purchase Windows Server 2012 or 2016 CALs.
- Remote users: If most of your users work remotely and are validated through a DC that is hosted in Azure, you will need to purchase Remote Desktop Services (RDS) CALs. These CALs are required for each user or device that accesses the server remotely.
- Software Assurance: Software Assurance is an optional add-on that provides additional benefits such as access to the latest version of the software and technical support. If you do not have Software Assurance, you will need to purchase new licenses for the new DC.
- Licensing strategy: A good licensing strategy for your scenario would be to purchase Windows Server CALs and Remote Desktop Services CALs for the 120 users in your domain. This will ensure that all users have access to the new DC, regardless of whether they are working remotely or in the office. If you are planning to use Windows Server 2019 or later, you can also consider using Azure AD Domain Services which is a service that allows you to use Azure AD as a domain controller for your on-premises resources, this way you don't need to buy any licenses for the new DC.