Oh, I don't know... I've got a Mac and PC, so I've never tried it. However, I suspect that it's one stereo pair per instance. I mean, gbit can handle lots of audio, so I don't see why it would be so limited. Also, I've tried apps like wormhole, and they do pretty well. Latency is an issue, though, at least with wormhole. But since the AU net plugs are Apple-made, I'd imagine any optimization possible has been done, so it would at least be worth a try.
Otherwise, get out there and drop some serious dough on hardware! I'd say RME is a pretty good bet!
cheers,
J.