noun, adverb, noun which may take the genitive case particle 'no'
Used as an adverb, noun, or の-modifying expression for a large quantity or number. 多くの is a common way to say 'many ...' in a slightly more written or general style than たくさんの.
多くの人が駅前に集まった。
Many people gathered in front of the station.
彼はその事件について多くを語らなかった。
He did not say much about the incident.
noun
Usually appears as Nの多く or 多くは to mean the greater part of a specified group or amount.
学生の多くは県外から来ている。
The majority of the students are from outside the prefecture.
adverb
mostly; mainly; usually; often
Adverbial use describing what is generally, chiefly, or often the case. It is somewhat common in explanatory or written style; 主に, たいてい, or よく may be more conversational depending on the context.
この地方の祭りは多く秋に行われる。
Festivals in this region are usually held in autumn.
多くの場合、事前の予約が必要です。
In most cases, advance reservations are required.
A clearer adverb for 'mainly' or 'chiefly'; use it when the intended meaning is not quantity but primary focus or basis.
Grammatically, 多く is the 連用形/adverbial form of the adjective 多い. The kanji 多 is conventionally associated with the meaning 'many' or 'much'; no further historical origin is asserted here.