What is the meaning of these words/sentences?

Question

I've some issues getting through the message of my language correspondence partner:

日本では、たくさんの花火大会があります。

７月には、神戸の花火大会、大阪の花火大会に多くに人が訪れます。

１万発 でとても迫力がありますよ。

あと、夏になるとセミがたくさん鳴いて、外はとてもうるさいです。

Content-wise, I understand what I'm being told here, but I don't understand how some things work and therefore also have problems determining what exactly is being said.

1) 多くに

Why is there a に attached? I think that 多い shall become an adverb here, but い adjectives become adverbs solely by replacing い with く.

I could imagine 多くに in context of a sentence like this:

人は多くになりました。

It became a lot of people/The people got numerous.

But first I don't know whether this assumption is correct and second I don't see whether なる and　訪れる　could be treated equally concerning this case.

2) １万発

What does this 発 mean? 万 is まん　but the other one I couldn't find a satisfying entry on jisho.org, especially not in connection to 発.

3) Since I don’t really know 万発 I have trouble interpreting the meaning and function of this で

4) 夏になるとセミがたくさん鳴いて

I don’t really know about this one. It's something about “when it becomes summer” and… “the half”???

I think it's about some half which sings loud, but I have trouble integrating the conditional clause meaningfully into the full sentence.

Answer 1

1) 多くに in 大阪の花火大会に多くに人が訪れます。

It is typo. It should be 多くの, so 大阪の花火大会に多くの人が訪れます means A lot of people visit Osaka to see the festival of shooting off fireworks.

2) １万発

発 is a counter to count fireworks. It is also used to count bullets or cannonballs. １万発の花火 is 10,000 fireworks.

3) since I don’t really know 万発 I have trouble interpreting the meaning and function of this で

１万発で implies １万発の花火が上がるので since there are 10,000 (shooting off) fireworks.

4) 夏になるとセミがたくさん鳴いて I don’t

セミ is a cicada, that is written as 蝉 in kanji.

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EDIT:

As for the meaning or function of で in １万発 で とても迫力がありますよ, naruto gave me a comment to my answer as "this で after 1万発 is the te-form of the copula だ", so I'll consider it further.

I quote the explanation of the te-form of the copula だ from here.

Te-form of copula (-です)

This form let adjectives connect to other adjectives or verbs.
"です" changes into "で" + another sentence.
In the past, we used these choppy sentences: 名前は、一郎です。　年は１８才です。 せんこうはアメリカ文学ぶんがくです。
If we use te-form of desu, these sentences can be connected smoothly as follows:
名前は、一郎で、年は１８才で、 せんこうはアメリカ文学（ぶんがく）です。 (lit. My name is Ichiro, age is 18 years old, and major is American Literature.)

Reading this explanation, "te-form of copula" shows that it has a function to connect plural sentences having the same subject simply by "and" into one sentence. Applying this rule to OP's sentence "１万発 で とても迫力がありますよ。", you could understand that there were two sentences at the beginning as "１万発です。" and "とても迫力がありますよ。" Because their subjects were omitted, by complementing the subject you'll get two senteces like "花火は１万発です。The fireworks are 10,000 shots" and "花火はとても迫力がありますよ。 Fireworks are very powerful and impressive."; or "打ち上げられる花火は１万発です。Fireworks launched are 10,000 shots" and "打ち上げられる花火はとても迫力がありますよ。 Fireworks launched are very powerful and impressive."

Here we can confirm that the rule of "the te-form of the copula だ" functons. Although this rule fuctions, it certainly seems that the combined sentence is monotonous because we merely joined the two sentences with "and".

So I interpreted the で in "１万発 で とても迫力があります" as the short form of なので that means a conjunction as "because or since" in the original answer. 　

With this interpretation, the two sentences become a causal relationship that makes the combined sentence lively, which the writer of the letter might have thought apart from whether the writer knew the grammatical rule or not.