Like plenty of other JRPG franchises, the numbers are just telling you how many games came before it.
There’s only one Disgaea game that builds on what came before, and that’s Disgaea D2: A Brighter Darkness, starring Laharl and Flonne from the original Disgaea games.
The rest of the Disgaea series can be played in any order, though we’d recommend starting with some of the more recent entries if you haven’t played Disgaea before.
Read more: Will Disgaea 6 be on PC? Not anytime soon
Disgaea 6 Is Coming Soon, But You Don't Need Any Previous Knowledge to Enjoy
Disgaea 2 has a PC port and is easier to find thanks to that, but unlike the recent repackaging of Disgaea 1 and Disgaea 4, it’s missing a few of the better quality of life improvements.
Disgaea 3 remains locked on the PlayStation 3 and PlayStation Vita for the time being.
However, Disgaea 1 Complete+ and Disgaea 4 Complete+ bring these two classic, older entries in the series in line with Disgaea 5, including streamlined combat, higher resolutions, and all previously released DLC.
You might recognize some of these characters in the Disgaea 6 DLC, which will continue the series’ tradition of adding new episodes built around older characters.
However, these are standalone episodes meant as callbacks for series fans, and you don’t need to play Disgaea in order to enjoy them or the base game.