They use modulation tricks to get multiple inputs on one audio channel. Sound-cards with more than 4 audio channels are not cheap, and if you gang multiple sou

They use modulation tricks to get multiple inputs on one audio channel. Sound-cards with more than 4 audio channels are not cheap, and if you gang multiple sound cards together you end up with clock drift. If you want more than 4 channels that don't drift from each other with consumer hardware, then you'll need to multiplex (And indeed, they offer non-multiplexing 4-channel options).