There are tons of good articles about this on internet- just stick to ones written for peer reviewed medical journals and hospitals - not the ones on blogs or forums written by other people like yourself that have read for 3 days longer on Google already. It ranges from as common as bipolar in some "expert" opinions to a completely invented diagnosis which is actually simply several other problems combined. The book and movie that made it popular in mainstream culture is known to be completely fake and the "Sybil admitted she faked the entire thing - and the very large number of people completely recanting their stories of it after a few years and admitting total awareness all the time is why many claim it does not exist at all.
You will find more casual and interesting articles saying it does exist and is common - and more actually scholarly and peer reviewed medical where it is not the person charging for treatment saying if it does exist it is exceptionally rare.
To me it does not really matter- in either case they are serious problems that need symptoms treated and since the treatment even by those that are vehement in saying it is common is to treat symptoms only so regardless of label is the same treatment protocol of therapy and no real medication support.