Ultimately, once all processing is finished, the end result that comes out of the speaker can be thought of as a single sound wave. During the mixing process, all the individual waves from all the different instruments are compiled together into effectively a single, complex waveform.

There's more to it, of course - most speakers have more than one speaker element (one for highs, one for lows, or more) and then you are of course dealing with multiple sound sources all coming together at the point where your ears are. And your ears only have one membrane (the timpanic membrane) that oscillates back and forth, so again this feed to your brain and nerves can be considered a single "source".

Ultimately, the amount of detail you can perceive depends on the relative loudness and position of different external sources, and the quality of the audio mix.