Is there a limit to how much nasal bones can narrow a nose? 

Sometimes, the limitation to narrowing the nasal bridge is due to soft tissues rather than bone support. If this is the case, narrowing of the nasal bones is not necessarily producing significant impact.
If the soft tissue envelope is intermediate in thickness or thin, it is possible to narrow the bridge by reducing the thickness of the nasal bones which is usually carried out relatively well with ultrasonic rhinoplasty. With ultrasonic rhinoplasty it is also possible to narrow slightly the bony part of the nasal septum which can also restrict the movement of the sidewalls.
All in all, when the soft tissue is not the limitation, the nasal bridge can be made relatively narrow. It is important to appreciate that there are limits to how good the nose would look when the nasal bridge is too narrow and this also needs to be avoided.