Typically there is only one microphone, one loudspeaker, or, in the case of headphones or multiple loudspeakers, they are fed from a common signal path, and in the case of multiple microphones, mixed into a single signal path at some stage.
Typically there is only one microphone or, they are fed from a common signal path, and in the case of multiple microphones at some stage. This was mixed into a single signal path. This was in the case of headphones. This was in the case of multiple loudspeakers. Only one microphone is one loudspeaker.