This fantastic video from a few years back does an excellent job putting historical and musical context around Tallis' magnificent "Spem in alium"! Definitely worth the watch to learn more and see the excellent visuals (though I don't think any recording, especially one made from home recordings during covid lockdown, can compete with the incredible experience of hearing or performing this live!
Spem in alium
How big was the scale of polychoral music? The biggest was a mass by Alessandro Striggio for 5 choirs of 12, totaling 60 independent polyphonic lines. Lassus, Malvezzi, Rossetto, and others wrote 30-50 part pieces. But the best known is Tallis’ Spem in alium for 40 voices (8 choirs of 5).