Translation:There are a lot of men.
There is a subtle difference between 多い [おおい] and 沢山 [たくさん]。多い can suggest that there is more than necessary, an almost extensive amount, or more than there was previously. たくさん is 'a lot' in the general sense.
If someone told me that a library has 多くの本 vs 沢山の本, I'd say the first is overflowing with so many books!
大勢 is only used for humans and sometimes animals! 多い and 沢山 must be used for objects. 多い carries same implication that 大勢 does, I think, but one is an adjective and the other is a noun.
Not sure why this explanation got downvoted... Maybe the use of kanji?
大勢 = おおぜい
沢山 = たくさん
多い = おおい
@ Rikkapika. Thanks for the explanation. おおぜい is a new bit of vocab for me (or I forgot it many years ago).
If you want to use おおい, the sentence should be like 男の人がおおいです。which means "There are many men among them."
No. おおぜい is a noun (which can also be used like an adjective if you type の after it, e.g. おおぜいのひと). おおい is an い-adjective.
大勢 is indeed a noun, but isn't it working adverbially here (with います)?
Just checking my understanding - but I don't think you could sub any other noun in for 大勢 in this sentence, could you? For instance, 男の人が犬います. wouldn't work - not only logically, but grammatically. It would have to be です. Correct?
ます indicates action, です indicates state of being, I believe. います indicates the action of existing in a certain place (I think, I'm not native). Using 多い with います would be incorrect in this context. You could use 多い in a different context, though (「ここに多い人います！」, "There are a lot of people here!"), but just be careful (「ここに多い人です！」, "Here is many people!").
Once again, diversity is a good thing, but I'm questioning if this expression is in JLPT N5 at all...? たくさん should cover that level of basics, I think.
In one lesson, the characters for "men" in this sentence meant boy, then it taught me new characters for man/men, now it tells me this means boy. Sometimes I feel cheated.