Two comments, well three actualy.
The MSA take a dim view of people driving cars on the road with competition numbers thereon when they are not actually competing at that time. Sprints/hillclimbs etc, different numbers used for every event so you have to change them anyway. Most people with dark coloured cars just put white numbers directly onto the (polished) bodywork, because they peel off easily after the event, and if you do it carefully and put it back onto the backing paper it can be re-used.(saves running out of a particular digit and have to run round buying more once the entry list has revealed your next number).
As for having roundels permanently on the car, they become shabby very quickly, which is why most people no longer use them.