This is the 8433rd most frequent Turkish word.
onlarca
"Onlarca" means "dozens of" in English.
In this sentence, 'onlarca' is used to indicate a large but unspecified number (dozens) of books in the bookshelf.
Kitaplıkta onlarca kitap vardı.
There were dozens of books on the bookshelf.
Here, 'onlarca' implies a significant quantity of reasons mentioned by the person.
Onlarca sebep sıraladı, hepsinin haklı olduğunu düşünüyor.
He listed dozens of reasons, and he believes all of them are justified.
In this context, 'onlarca' is used to associate the event with a considerable number of participants.