The TELNET TCP connection is established between the user's port U and the server's port L. The server listens on its well known port L for such connections. Since a TCP connection is full duplex and identified by the pair of ports, the server can engage in many simultaneous connections involving its port L and different user ports U.
When used for remote user access to service hosts (i.e., remote terminal access) this protocol is assigned server port 23 (27 octal). That is L=23.