How many books are there to la corda d oro?

There's no single book called "La Corda d'Oro." It's likely you are referring to the *La Corda d'Oro* series, a popular Japanese light novel series with a manga adaptation and several anime adaptations.

The *La Corda d'Oro* light novel series has 21 volumes written by Yuki Kure.

It's important to specify which media format you're interested in (light novel, manga, anime) as each adaptation has its own number of volumes/episodes.