They aren't common - you don't see many normally - but people do have them, in fact you can see one on YoUTube. It's not possible to get exact figures as a mixed dog, which isn't an official breed, doesnt have numbers the way a pedigree dog would. It doesnt seem to be on the official American hybrid list, for instance, though there are pitbull mixes and husky mixes there - just not the two together.
I have never seen one. I have volunteered in Bay Area dog shelters and have never run across such a mix.