The complete series. Of course, in addition to having trophies/achievements for both the core multiplayer modes as well as Blackout, Black Ops 4 also brings back Treyarch’s iconic zombies mode. So, there are plenty of achievements to obtain by playing that, too.
Since there’s much more content on the way for Black Ops 4 (notably coming as part of the season pass, though presumably not all of it will be free), especially for the zombies and Blackout modes, which Treyarch promises to routinely update throughout the game’s life cycle, it’s possible that more trophies/achievements will be added to the game down the line. But, for now, there are plenty of challenges already on the docket for players to try to attain now that the game is out now on store shelves.