Learn Turkish: Onun numarası kaç?

The base word for "number" is numara. Because we are talking about "his" number, we need the 3rd person singular possessive suffix. Since numara ends in a vowel, we must add the buffer consonant s before adding the suffix ı (which is chosen by 4-way vowel harmony). So, numara + s + ı becomes numarası.

The word Onun already means "his", so why do we still need the suffix on numarası?

In Turkish, possession is always marked on the owned item itself, even if you already explicitly stated the owner. You can think of the literal translation as "Of him, his-number". The owner word (Onun) is actually optional and often dropped, but the possessive suffix on the owned item (numarası) is absolutely required!

Why does this sentence use kaç ("how many") instead of ne ("what")?

In English we ask "What is his number?", but in Turkish, you almost always use kaç (how many / how much) when asking for things that will be answered with digits, such as phone numbers, shoe sizes, or ages.

Where does the first n in Onun come from?