The range of ports that can be used for RDP on Windows Server 2016/2019 is typically defined by the TCP/IP protocol. Typically, the range includes ports from 1024 to 65535. However, you should keep in mind that certain ports may be reserved for other services or used by you on the server for other purposes.

When choosing a port to replace the standard RDP port (3389), it is recommended to use ports above 1024 to avoid conflicts with other services. It is also important to make sure that the selected port is not blocked by a firewall or other network devices on the connection path.

Please note that after changing the RDP port, you must specify the new port when connecting to the server using the RDP client.