Tonight I went to a free concert at the Cathedral (who can argue with a free concert?) - a bunch of really shockingly good local choirs performing works by Thomas Tallis, a 16th century English composer. The part I was most excited about hearing was the 40-part motet Spem in alium. Yes, that's 40 distinct parts.
It turned out not to be as good as when I heard it done a couple of years ago at Stanford. But then that was the Tallis Scholars, so you'd expect them to be good at singing Tallis.
What makes this piece work and not totally fall into disarray is that the 40 parts are divided into eight choirs of five. Most of the time only one or two choirs are singing at a time, and the sound moves back and forth across the group. But at a few points in the piece, everyone sings together, producing this unified mass of sound.
I recommend going to see it sometime. Sit up front.