Many; much; plenty — used as a noun-like expression to indicate a large number or amount (as in 'many of ...').
多くの学生がその講義を受講している。
Many students are taking that lecture.
会場には多くの人が集まった。
A large number of people gathered at the venue.
The majority (of); the greater part — indicating the larger portion within a group.
意見は多くが賛成だった。
The majority of opinions were in favor.
投票者の多くは賛成票を投じた。
Many of the voters cast votes in favor.
As an adverb: mostly; mainly; usually; often — indicating frequency or tendency.
仕事は多く夜に行うことが多い。
Work is often done at night.
彼の成功は多く努力の賜物だ。
His success is mostly the result of hard work.
多くの場合、準備が必要だ。
In most cases, preparation is necessary.