maybe you’re confusing between MIDI chanels and tracks. The MIDI channel is a property of any MIDI event sent through a MIDI connection. You can have a single MIDI track receive input from all the 16 available MIDI channels. If you have 5 physical MIDI controllers each sending to a separate MIDI channel, you can set the n-Track’s track to receive input from all the available MIDI devices. The synth will then receive all the device’s MIDI events, each one with its original MIDI channel.
Make sure that all the MIDI controllers are selected in the Settings -> MIDI devices box, the track must be set to receive input from “All channels, all devices”, and also set the track’s MIDI channel to ‘(none)’ in the MIDI track properties box, that appears right clicking on the track and selecting ‘Properties’ from the popup menu. If the MIDI channel is set to say ‘5’, n-Track will remap each event to have the selected channel 5. If it’s set to ‘(none)’ no remapping will take place.