几十 + Measure Word + Noun
The expression 几十 (jǐshí) means "tens of" or "several dozen" and indicates an approximate quantity in the range of ten to ninety-nine. It is used when the exact number is unknown or unimportant. For example, 几十年来 means "over the past several decades."
这家公司已经经营了几十年。
Zhè jiā gōngsī yǐjīng jīngyíng le jǐ shí nián.
This company has been operating for decades.
几十个学生报名参加了这次比赛。
Jǐ shí gè xuéshēng bàomíng cānjiā le zhè cì bǐsài.
Several dozen students signed up for this competition.
他收藏了几十本关于历史的书。
Tā shōucáng le jǐ shí běn guānyú lìshǐ de shū.
He has collected dozens of books about history.