Learn Turkish: On üç defter açık. - Thirteen notebooks are open.

This is a strict rule in Turkish: nouns always stay singular when they come right after a number. You say on üç defter (literally "thirteen notebook"), and adding the plural suffix -ler here would be incorrect.

Where is the word for 'are' in this sentence?

There isn't one! In Turkish, you don't need a separate word for 'is' or 'are' in simple descriptions. Just putting the adjective açık (open) at the end of the sentence is enough to mean 'are open'.

Does the word açık change if we change the number of notebooks?

No, açık stays exactly the same. Even if it were one notebook (bir defter açık) or a hundred (yüz defter açık), the adjective describing them doesn't need any special plural endings to match.