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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?"
Consider What colour bus is that? against What bus colour is that? The former would be answered "It's a green one [which means it belongs to Green Bus Company, and is not the one you want]" because the noun is bus and colour is an adjective of sorts. What bus colour is that? would have to be answered "Shade A75 Soothing Green" (though I'm having difficulty imagining a situation where anybody cared) because the noun is colour and bus is an attributive adjective. It is exactly the same with number; that is a number 11 bus, but its bus number is on a little chassis plate for the mechanics' benefit.
If we speak of a number 3 bus or a number 8 bus, then it follows that we ask ‘What number bus is that?’
A house number, ticket number, seat number or phone number is a fixed property of the entity to which it is attached. In the same sense a bus will have a bus number (identifying its unique combination of chassis and body).
If I am looking down the road at an approaching bus, I am usually not interested in the mechanical details of the vehicle. The information I want is the path it will follow as indicated by the route number which is temporarily displayed on this bus.
When I ask "which number bus" is coming, I mean what is the route number of this bus. That is quite different from its bus number.
Actual mini-conversation I've had at bus stops:
"Which bus is that?"
"It's the 22. And a 36 behind it." (for bus route 22 and 36)
You're waiting for a bus route, not a specific bus on the route (usually). No one is going to think you want a bus's unique identifier - generally, no one except drivers and mechanics care about that.
Alternatively, if you want to specify what type of number, I'd ask:
"Which bus route is that?"
Both versions are perfectly natural English, but in fact the "superficially less grammatical" form what number bus is several times more popular than what bus number.
I imagine the reason for this is that in the context of asking the question, number is a far more important word than bus, so you want to attach it more closely to the "question" indicator what.
You could say "what bus number is that?", but you normally don't really care what the number is, what you really care is what bus it is, (if I plan to take the number 15, I have no interest in 15 being on a bus, other than the fact that a bus with 15 on it, is a number 15 bus, and will follow the number 15 bus route). The number is just a way to inform you of that, so "what number bus is that?" asks directly about what you most care about.
Both amount to the same thing, in ultimate information if not in how it is enquired about, and both are valid.
