Literally, the sagas were long, epic poems written in Iceland in the Middle Ages, containing histories of royal families and heroes and gods.
By extension, a saga is usually a long literary work containing a multigenerational family history, and usually written in a serious style.
I think "multigenerational" is the key, not the length of the series. Ken Follett's series of "Kingsbridge" books is called a "saga' because it deals with several family generations.
British author John Galsworthy wrote a three-novel trilogy to which he gave the name "The Forsyte Saga" because it covers the story of several generations of the Forsyte family.
Conversely, The Harry Potter series isn't a "saga" because it just deals with Harry Potter's life. Similarly, Patrick O'Brian's TWENTY-volume series of seafaring novels, the "Aubrey/Maturin" series, is not a saga because it just deals with the seafaring career of one person, Captain Aubrey.