The Disgaea series didn't really have all that much to do with Nintendo platforms until the Switch came around, and now there are multiple installments available. There's no doubt those versions have racked up some sales, and have helped Nippon Ichi reach a new milestone.
According to the latest report, the Disgaea franchise has hit 4.5 million units sold worldwide. That includes physical and digital sales. With Disgaea 6 just around the corner, the series should be able to break the 5 million unit mark no problem.