How commonly does a dilated cardiomyopathy occur?

Depends. It is not that common but can be caused by various reasons including childbirth, infection or drugs to name the most common. In most cases patients recover or can be treated by medications. Make sure you find a good doctor who specializes in this.
8 per 100, 000. There are approximately 8 cases per 100, 000 people. Roughly 20% of dilated cardiomyopathy are familial.