I would put 3-4 microphones on the 20 singers, and 2 on the piano (low and high), and record into protools.
I would get:
protools mBox pro
an external hardrive and firewire
2 shure sm57 mics (for the piano)
3 to 4 shure sm58 mics (for the vocals)
I believe you can record 6 separate lines with the mBox pro which would allow you to take all those sources and track independently and then be able to mix after the fact using the protools software on your mac. You should investigate that first though. Another option would be to get a small mackie mixing board and do a live mix of all your vocal microphones and run it into protools as a stereo feed to 2 tracks, and run the piano in on 2 other tracks (so using 4 instead of 6 tracks).
good luck, clearlythere!