noun
large group; horde; large crowd/herd/flock/school/swarm
Refers to a large gathering of people, animals, or things. The specific English word (crowd, herd, flock, etc.) depends on what is being described. Often used with の to modify a noun.
駅前に大群の人が集まっていた。
A large crowd of people had gathered in front of the station.
空を大群の鳥が飛んでいた。
A large flock of birds was flying across the sky.