Complex sentence: Mixers offer a multitude of inputs, each is fed by a track from a multitrack recorder; mixers would normally have 2 main outputs (in the case of two-channel stereo mixing) or 8 (in the case of surround).
Split into simpler sentences: Mixers offer a multitude of inputs, each is fed by a track from a multitrack recorder. Mixers typically have 2 main outputs (in the case of two-channel stereo mixing) or 8 (in the case of surround).