This is the 1438th most frequent Chinese word.
几十
"几十" means "tens" or "several tens."
Here, "几十" is used to represent an unspecified but considerable number of years, generally between 20 and 90.
他已经工作了几十年。
He has been working for dozens of years.
In this sentence, "几十" indicates a variety of books in the store, generally suggesting a range from 20 to 90 items, giving an approximate idea of quantity without providing a precise number.
商店里有几十种不同的书。
There are dozens of different books in the store.
"几十" here specifies a requirement of several tens of people, emphasizing an approximate number within the '20 to 90' range.