Learn Turkish: Kütüphanede çok kitap var. - There are many books in the library.

In Turkish, when you use a word that tells you the quantity of something—like numbers, çok (many/a lot), or the question word kaç (how many)—the noun that follows stays in its singular form. So, you must say çok kitap instead of çok kitaplar.

If I wanted to ask 'How many books are in the library?', how would I use kaç here?

You would replace the word çok with the question word kaç. The sentence becomes Kütüphanede kaç kitap var?. Just like with çok, the word kitap remains singular after kaç.

Does the word var change to show that there are multiple books?

No, var is very flexible. It simply means that something exists. Whether you are saying 'there is a book' or 'there are many books', the word var stays exactly the same.

What does the -de at the end of kütüphanede do?

It is the locative suffix, which tells us where something is located (meaning 'in', 'on', or 'at'). Adding -de to kütüphane (library) turns it into kütüphanede (in the library).