Final answer:
To see how many concurrent telnet sessions the IFT MAIN router can handle, use the 'show line' command on a Cisco router, which displays used and free TTY lines. The number of sessions depends on the specific router configuration and model, and adjustments can be made in the configuration mode using the 'line vty' and 'session-limit' commands.
Step-by-step explanation:
To determine how many concurrent telnet sessions you can run on the IFT MAIN router, a commonly used command (assuming you have appropriate access and the router is running Cisco IOS) is show line. This command will list the lines and indicate how many are in use and how many are free. For example, it will show the TTY lines which are used for incoming telnet sessions. The number of concurrent sessions allowed will depend on the router model and its configuration. In Cisco routers, this can often be monitored or set by looking at the number of VTY (Virtual Teletype) lines configured which usually ranges from 0 to 4 or 0 to 15 by default.
If you need to change the maximum amount of telnet sessions, you would use the configuration mode command line vty 0 15 and then set the session limit accordingly within that range, using the command session-limit number.