The way it seems to have been done traditionally, as you note, is that everything is lumped together as "The Henry Plays," which is rather unfair to the Ricardians among us! Although on the other hand the only histories fics that ever get written for Yuletide are Henry V/Scroope for some reason.

I think that we are probably stuck with "The Henry Plays" as a category because it already exists, although if I were in charge of these things, I would count the individual plays individually but consider the multipart plays one play for categorization purposes. I suppose that the Richard plays would probably each get their own category if anyone wrote anything about them for Yuletide; other Shakespeare plays get listed individually...