This is the 101st most frequent Japanese word.
多く
"Many" or "a lot."
Here, '多く' is followed by 'の人' (people), forming '多くの人', which translates as 'many people', used to describe the quantity of people visiting.
多くの人がこの公園を訪れます。
A lot of people visit this park.
Here, '多く' is used before the noun '本' (books), typically constructed as '多くの本', meaning 'many books'.
彼は多くの本を持っています。
He has many books.
In this sentence, '多く' is used as an adverb modifying the verb '悩んだ' (worried), indicating the extent of the action, meaning 'a lot'.