it all depends on how and what you have setup to record (input buses) and playback (output buses);
duplex buses are just a convenience, nothing more than when you have the same channel count on both end, otherwise you should make it all match the channel count for each of your physical sources (mics, guitars, synths, turntables, decks, etc.) or software ones like any JACK client app. the same applies to outputs (speakers, processors, etc.)
the Master bus being duplex and stereo (2 channels) is just a "factory" default convenience; you don't really need to have all your available physical inputs and outputs grouped in some buses; you only need to use the sets that are meaningful to the session that you're working at a given moment.
there are too many ways, reasons and options to do this and that, for example, if you want to record your voice from a mono source (eg. a microphone) then you ought to create a mono (1 channel) input bus and only: there's no reason whatsoever for it being duplex or stereo there.
it all depends on how and what you have setup to record (input buses) and playback (output buses);
duplex buses are just a convenience, nothing more than when you have the same channel count on both end, otherwise you should make it all match the channel count for each of your physical sources (mics, guitars, synths, turntables, decks, etc.) or software ones like any JACK client app. the same applies to outputs (speakers, processors, etc.)
the Master bus being duplex and stereo (2 channels) is just a "factory" default convenience; you don't really need to have all your available physical inputs and outputs grouped in some buses; you only need to use the sets that are meaningful to the session that you're working at a given moment.
there are too many ways, reasons and options to do this and that, for example, if you want to record your voice from a mono source (eg. a microphone) then you ought to create a mono (1 channel) input bus and only: there's no reason whatsoever for it being duplex or stereo there.
seeya