Reason's Hardware Interface can accommodate up to 64 channels of MIDI, divided into 4 buses, each with 16 MIDI channels. There are two basic ways you can route incoming MIDI to Reason devices, which is set in the Preferences - MIDI and Advanced MIDI dialogs:
If you choose this option, the selected track's destination device automatically receives incoming MIDI data. This means that you only have to send MIDI over the same port and channel as the sequencer is set to use (in the MIDI Preferences), to access any audio device in Reason. This is the easiest way of routing MIDI if you are using the built in sequencer. There are no settings you need to make in the Hardware Interface if you use the Sequencer input.
By using the MIDI "External Control" inputs.
This is set in the Advanced MIDI Preferences. You can select up to four buses (if your MIDI interface supports it), each with 16 MIDI channels. If this mode is used, you use the pop-up menu for each MIDI channel in the MIDI In device to select the destination device you would like to route the MIDI to. If you want to send MIDI to Reason over several channels simultaneously, you have to use the external control inputs.