I rudeboy, the cathedral was designed to accommodate 40,000 worshipers.[http://www.bluffton.edu/~sullivanm/milancath/duomo.html]
Owl_Eagle is correct: They probably would killed a lot more than dozens - had they been successful.
I stand corrected. I’ve been in that cathedral a number of times, and there’s no way I can be convinced that it’ll fit 40,000—but that’s just my opinion.