Hi Kooyeen, I was about to vote for "perfectly okay," and then realized that I really couldn't imagine saying "I saw many people" in a casual conversation. I'd say "a lot of" or "lots" or "tons" or whatever. ) but you're right, it definitely sounds more formal than casual.
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Grammar Geek...and you won't stick out like a sore thumb (do you have that idiom in your repertoire yet?) but you're right, it definitely sounds more formal than casual.I didn't know that idiom, but I guessed the meaning anyway. I see that "many" could sound more formal thna usual in certain cases, but it's not really "formal" or "too formal".