I don't have data, but I assume it's just as common as in the general population. I remember reading anecdotes about perfumers not smelling certain things, but I don't remember the specifics.
Note that the point is not that one person is more anosmic than another in general, it's just that each of us has his or her own specific anosmias to specific musks. I guess this will certainly affect a perfumer's choice of the musks he uses, either because he would prefer to use another musk, or because he might be led to overdose. In the past, I think perfumers would use a mix of musks to avoid this.
I'm also not sure how the fixative property of musks work. That is, even if one doesn't smell a particular musk, he may still smell the other materials that the musk helps fixate. So in a way the musks still do have an effect.