Imagine you are at the bus stop. You see a bus coming, but you can't read the number, so you ask: "What number bus is that?"
However, usually the noun used as a determiner precedes the main noun, for example: house number, ticket number, seat number, phone number, etc. So why, in this case, don't you say "what bus number is that?"
By the way, my English teacher told me the right way to say this is, "What number bus is that?"