I don't know what it's called but I refer to it by its suffix, a -logy or a set of compound works. From how the internet describes it it can be one singular story spanning over several novels or separate individual stories, but I like the first part of the definition when describing a series of books that has one core plot spanning over several consecutive novels. Compound Sets: 1.) Standalone 2.) Duology 3.) Trilogy 4.) Tetralogy or Quadrilogy (if you're Ridley Scott) 5.) Pentalogy 6.) Hexalogy 7.) Heptalogy 8.) Octalogy 9.) Ennealogy 10.) Decalogy 10+.) Polylogy (but I also read that this can be used to describe a set of two or more works, so this is more of an umbrella term) If any of this is wrong feel free to correct me.
Piers Anthony would disagree a bit. 27 books is a Trilogy (of course it works when your MC's magical power is that magic doesn't affect him.) Anywho, how many past Trilogy, don't simply fall off the continuity/plot train? Also isn't anything longer than a Trilogy a series? And are not Series written by people who don't know how to end a story?
After trilogy, I cannot see myself using any word other than just series. Frank Herbert's universe of Dune is a series of six books. The latter books published by Frank's son, Bryan, are abomination fit only for the ministrations of a gom jabbar.