Do lots of people get muscular dystrophy, or is it unusual?

Relatively common. The incidence of duchenne md is 1 in 3500 male births, with 1/3 of the cases caused by new mutations. Another md, myotonic md has an incidence of about 13.5/100, 000 live births. Most of the other muscular dystrophies are much less common, are frequently genetically transmitted or regional clustered (oculopharyngeal md in french canadians) or are sex linked(emery-dreifuss).