I think it's because it can be interpreted in two ways depending on what answer you want. Most of the time this relationship between x and y is obvious. Airplanes don't own airports. If you want to know how many people who's got blue eyes you'd ask "Hvor mange folk har blå øyne? Mange", if you want to know how many blue eyes people got you'd ask "Hvor mange blå øyne har folk? Mellom null og to".
"Hvor mange flyplasser har fly?" All of them, of course.