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crowd of people; great number of people
in great numbers
「大勢」 has another reading with different meanings.
大勢の人が集まっていた。
A large crowd had gathered.
大勢で食べると料理がもっと美味しい。
Food tastes better when eating with a crowd.
花火大会には大勢の人が見に来ていた。
Many people had come to watch the fireworks festival.