As I'm the third answerer and two of them who answered ahead of me explained well already, I'm just gonna add some supplementary explanation. ・ロサンゼルスには、日本人が沢山居ます。 This is like "Los Angeles (has) got Japanese a lot." This one is what Ueda san has explained, which is a sentence that doesn't have の in it. This one is the usage of the adverb. It feels that the subject (the main topic) is Los Angels with this sentence. ・ロサンゼルスには、沢山の日本人が居ます There are many (a lot of) Japanese in Los Angeles This one is the one that iyuuki has explained. With this sentence, the focus point seems more 沢山の日本人 and a fact itself that there's lots of Japanese in Los Angeles. It makes this effect because the word 沢山 qualifies a noun 日本人 as an adjective, so it highlights a noun 日本人. As a longer noun: 沢山の日本人 than just a noun 日本人. Both are grammatically correct and sound natural. It might be depending on sentences or context which to choose.

Other than that, you usually don't use a particle "で" after "ロサンゼルス". You could use "は", which would be a so-called topic marker in the sentence. You more often use "に" or "には", which both mean almost the same as "in" in English.

You don't necessarily need to add "の" at the end of "たくさん". ／If you add ”の”, it makes an adjective "たくさんの" together with "たくさん", which modifies a noun "日本人". 〔たくさんの日本人〕／If you don't add it and leave "たくさん" as it is now, it makes an adverb all by itself, which modifies a verb "います". 〔たくさんいます〕

ロサンゼルスで"は"たくさん"の"日本人がいます。 is more polite and better. Because it means Los angels has a lot of Japanese and ロサンゼルスで"は" represents a subject. For me, ロサンゼルス"には"たくさん"の"日本人がいます。seems to be best.