translation - How is the adjective 多い being used as a noun 多く here?

I know 多い means 'many' but I don't really understand how it's being used in the sentence below as a noun 多く.

多くの人々がすべてを失い、多くの人が亡くなった。

My best translation - 'Many peoples have lost everything and many people have come to lose someone'.

I'm not happy with my translation either.

Answer

Both　「[多]{おお}い」 and 「多くの」 mean "many" but their usages are completely different.

Grammar in terms of parts of speech:

「多い」 is an adjective all by itself.

「多くの」＝「多く」 + 「の」. 「多く」 is a noun meaning "plenty" and because it is a noun, it needs to be followed directly by 「の」 to function like an adjective.

Usages:

To express "many (noun)", one can say 「多くの + noun」, but one cannot say 「多い + noun」. I have met 多くの Japanese-learners who did not know this.

The correct and natural usage of 「多い」 is in the form 「(Noun) + は/が + 多い」.

Sentence in question:

「多くの[人々]{ひとびと}がすべてを[失]{うしな}い、多くの[人]{ひと}が[亡]{な}くなった。」

Your translation of the first half is good except for the "many peoples" part. It should be "many people". 「多くの人々」 means the exact same as 「多くの人」.

「[亡]{な}くなる」 is a euphemistic word for 「[死]{し}ぬ」 = "to die". Thus, one can think of it as the Japanese counterpart for the verb "to pass".

"Many people have lost everything and many (people) have passed away."