Many professional mixing desks have many channels (usually between 8 and 72 in banks of 8). A channel controls 1 input source (microphone or guitar etc). Each channel will have at least an input gain (turn up the input volume), and a fader (turn up or down the output volume). In between may be equalisation (low, mid, high), multiple send and return (push some of the signal to another channel or outboard device and the modified signal back to the channel). Then you have mute (turn off the whole channel), and solo (turn off or reducebthe volume of all the other channels). There will generally be one big dial that controls the master volume too.
Multiply that by 72 channels (big pro mixing desk) and you have hundreds of knobs and sliders.
Why 72?
When recording drums you may have between 12 and 20 microphones. Then 2 or 3 for each guitar (1 direct plus 1 or 2 microphones on the speaker). 2 for bass (direct plus speaker). 1 for main vocal. Plus others for other instruments. This is for a live or tracking recording. Then overdubs (adding solos or backing voxals) need separate channels for both playback and recording.
Then effects (delay, echo, reverb etc) may be separate to the channel send return.
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