I checked in some changes that enable Audacity to record more
than two channels. You can set the number of channels to
record in the Audio I/O preferences.
Clearly the UI is still limited, but I didn't see
any reason not to enable support for more than two channels
for bleeding-edge users.
Of course, this assumes that you have a cool sound card plus a
version of PortAudio that supports it. I hacked up PortAudio
v18 for OS X to work with my USB Quattro, but I haven't checked
it in, since now it ONLY works in 4-channel mode and stereo
is broken. :) But I'll straighten it out soon, and eventually
help work on PortAudio v19 for OS X.
As soon as Josh ports AudioIO to PortAudio v19, though, we
should be able to create Windows and Linux/ALSA builds that can
do multi-channel input.