Adzeth: http://en.wikipedia.org/wiki/List_of_Mega_Man_video_games /edit: if someone doesn't want to click the link, that Wikipedia article says that there are 128 titles in the Mega Man series
Yeah, but that was written with usual Wikipedia eye for attention.
If you check the official data that is linked in that article you can see that that is incorrect, they are talking about sales units and that probably counts every separate release (so the same game for multiple systems as well).
I'm reasonably certain the Lost Planet series does not consist of 13 titles.
If you count spinoffs and such as part of a series, Mario is a good contender as is Castlevania. If you only count direct sequels.. Final Fantasy would be my guess.