What do they call 6 books in a series?

Answer 1

Although the term is not in common usage, the appropriate term is "tetralogy," which has the same derivation as "trilogy," for 3 publications.

For what it's worth, "Tetra" means 4, and the suffix "logy" refers to a publication.

Answer 2

I think it's a saga, like the Star Wars saga (there were 6 movies)

Answer 3

A series of 6 books is typically referred to as a hexalogy.

Answer 4

An Octology