Audition CS5.5 can open, edit and save multi-channel files (I haven’t tested the maximum channel count it works for); but it can’t create or record them, and if they are used in the multitrack view, they get folded down to stereo somehow (unless they are 6-channel, which is treated as 5.1). This is acknowledged to be work in progress, and more complete facilities are expected in the next version or two. At least I now have an affordable way to do spectral editing on my normal four-channel files.
SoundForge handles files of up to 64 channels - eighteen is simply the number of speaker positions that MS have thought of, not a limit on the number of channels in the format. MS say that speaker positions beyond 18 are reserved, but only because they haven’t defined them; but it is possible to use the WAV format with a different GUID and define the channel mask to mean something completely different, as is done in .AMB files containing ambisonic channel sets (NB: SoundForge rejects these because of the GUID; Audition recognises the GUID and opens them with a warning, but writes back with a standard GUID).