There are four main novels in the original Twilight series by Stephenie Meyer, commonly referred to as the Twilight Saga. In addition, there are a few related works that expand the universe. Direct answer:

• The Twilight Saga consists of four primary novels: Twilight, New Moon, Eclipse, and Breaking Dawn.

• In the broader Twilight universe, there are two companion novels and one novella that expand on or reinterpret the central storyline: The Short Second Life of Bree Tanner (novella/short story), Life and Death: Twilight Reimagined (novella-length reimagining), and Midnight Sun (Edward’s POV companion to Twilight). Some lists also count additional tie-ins like Bree Tanner’s novella as part of the extended universe.

If you’d like, I can clarify the exact official/commonly accepted counts for “in the series” versus “in the universe” and list each title in reading order.