This piece, Spem in alium, by Tallis, contains 40 distinct voice parts. Forty voices.
For those of you who haven’t sung in a choral context or aren’t familiar with the terminology, a standard mixed choir would sing pieces in 4 voices (musical lines), or maybe challenging piece in 6 or 8. This one has forty. Specifically, it’s scored for 8 choirs of 5 voices. 8 full choirs all singing different music to form one larger work. Definitely over the top, and definitely too cool to pass up.
Also, the score is available here if you like to read along.