No client access licenses (CALs) are required.
With Remote Desktop Services (RDS), one of the core virtualization technologies available in Windows Server 2019, Microsoft is progressing in its vision to provide the best virtualization platform for accelerating and extending desktop and application deployments from the data center to any device. In addition to the traditional session virtualization scenario (formerly known as "Terminal Services"),
Remote Desktop Services is expanding its role to provide an extensible platform for a Virtual Desktop Infrastructure (VDI). Remote Desktop Services allows a user to access applications, data and even an entire desktop running on a remote computer over a network. The client access device can either be a full rich Windows personal computer, or a thin client (such as Windows CE device).
Remote Desktop Services works by allowing individual applications or an entire desktop to run on a server, rather than on the user's workstation. Remote Desktop Services simply sends screen images to the users, and the user's machine in turn sends keystrokes and mouse movements back to the server. By doing this, Remote Desktop Services allows clients to run applications or desktop environments that they might otherwise not have the hardware or bandwidth to support.
On the server, applications and desktop environments can either run as shared sessions, or in the context of a virtual machine environment (aka virtual desktop infrastructure or VDI). An External Connector grants access to the server by an unlimited number of external users such as business partners, suppliers, customers, retirees, and alumni.
With this license you do not need to buy individual CALs. It is licensed per server.