From Dr. Deb's post about Dissociative Identity Disorder (commonly called Multiple Personality Disorder):
There are no precise numbers on how many people experience [Dissociative Identity Disorder]. Many who have the disorder don't seek treatment for fear that they will not be believed. But Stanford University psychiatrist Dr. David Spiegel - who has been studying this disorder for over three decades - estimates that DID affects about 1 percent of the population.
It all depends on how many times you count each case.