I drive a Volvo, each important rear light has a minimum of 2 bulbs, so 4 rear light bulbs, 5 brake light bulbs
the car tells me if one fails, but I am still showing a brake light or rear light until I can replace the bulb, Now I don't know if they are allowed to pull you if one bulb of two has failed, I would guess probably not as you are still displaying a legal indication.
That said I carry a set of spare bulbs (as I do pop over to france where a full spare set is a legal requirement), but if a bulb goes as you are driving on the motorway its not always wise to stop immediately to change it, much better to find a safe place to pull over (services).
Grendel