Learn French: Est-ce que tu as neuf ou dix cahiers ? - Do you have nine or ten notebooks?

The pronunciation of numbers like dix (and six) changes depending on the word that follows them! Because dix comes right before cahiers, which starts with a consonant, the 'x' becomes silent. If you said dix on its own, it would sound like 'deess'.

Doesn't neuf also mean 'new'? How do I know which meaning to use?

The French word neuf does double duty: it is the number 'nine' and also the masculine adjective for 'new'. You can tell it means 'nine' here because it is grouped with ou dix (or ten) and placed before the noun. When used to mean 'new', it almost always comes right after the noun instead.

Why does the sentence start with Est-ce que instead of just asking Tu as neuf ou dix cahiers ?

Both are correct! Est-ce que is a very common question starter. The phrase tu as means 'you have', and putting Est-ce que in front acts as a signal that a question is coming, much like 'Do' in English. While you can just say Tu as and raise your voice at the end, adding Est-ce que makes it extra clear.

Why do we use tu as instead of vous avez?