Learn Dutch: Wat is jouw telefoonnummer? - What is your phone number?

Both mean "your", but jouw is the emphatic form. You use jouw when you want to put special focus on the word "your" (like asking "what is your number?" as opposed to someone else's). If you don't need to emphasize it, you can just use je.

Why is telefoonnummer written as one long word?

In Dutch, when you combine two nouns like telefoon (phone) and nummer (number) to make a new concept, they are always glued together into a single word without any spaces.

Why does the verb is come right after the question word Wat?

This is the standard rule for open questions in Dutch. The question word (Wat) takes the first spot, and the verb (is) must come immediately after it in the second position.

Can I just say nummer instead of telefoonnummer?

Yes! Just like in English, if the context is obvious, you can drop telefoon and simply ask Wat is jouw nummer? (or Wat is je nummer?).