I think the audio is encoded into the RRD file. They may make a piece of hardware that has 12 discreet audio inputs, but I would be inclined to think this would be a software encoder. The great thing about tapeless formats is that you are not necessarily required to "lay back" anymore. Think Apple Compressor: one video file, six audio files and blam encoded video with surround sound. Or maybe it's just my fascination with getting rid of cables
As for the question about need:
If you were delivering to a distributor you could have 6 discreet 5.1 tracks and 6 discreet 5.1 M&E tracks.
-or-
Eight discreet 7.1 tracks and four LCRS tracks
-or-
Two sets of 5.1 tracks: one in english, one in french.
The possibilities are endless. There can never be enough audio tracks on a delivery/exhibition format in my opinion because for every one version of picture there are (in most instances) at least three different audio formats that must be delivered alongside it. One master is much easier to keep track of than multiple masters.