“Once, it was the ultimate weapon… A grand spell that required 3,333 praying monks gathered in a cathedral. It was like running sunlight through a magnifying glass to increase output.” Stiyl gritted his teeth. “Though this’s a replica with only 2,000 students, as the saying goes in this country, ‘pile up sand and you can build a tower.’ Its power can’t be underestimated.”
[...]
“Not quite,” Stiyl said, still facing forward. “The emphasis is in the core. The Gregorian Chant requires the simultaneous controlling of 2,000 people or it fails. If we can find the core of the 2,000 people and destroy it, we’ll end the Gregorian Chant.”

Toaru Majutsu no Index vol.2 ch.2 pt.5