Counting them up feels strangely satisfying: there are nine main novels in Diana Gabaldon’s Outlander sequence. The core titles in publication order are 'Outlander', 'Dragonfly in Amber', 'Voyager', 'Drums of Autumn', 'The Fiery Cross', 'A Breath of Snow and Ashes', 'An Echo in the Bone', 'Written in My Own Heart's Blood', and 'Go Tell the Bees That I Am Gone'. Those nine make up the backbone of Claire and Jamie's saga from the 1700s to later centuries.
Beyond the main novels, Gabaldon has added novellas, short stories, and companion volumes—stuff like the various Lord John tales and the companion guides that dig into history, music, and genealogy. If you want to follow the story chronologically or dip into side-stories, those extras are great, but the nine novels are what people mean when they talk about the series. I still get a kick thinking about how immersive the world is and how each book keeps pulling me back in.