As of 2018, 11 movies had been released under the Halloween franchise.
The series began with John Carpenter's Halloween in 1978, which introduced masked killer Michael Myers and survivor Laurie Strode. The next two films continued the original storyline, while Halloween III: Season of the Witch took an unusual detour by telling a largely unrelated story about dangerous Halloween masks rather than focusing on Myers.
The franchise later developed several timelines. Halloween 4, Halloween 5, and The Curse of Michael Myers continued one continuity; Halloween H20 and Halloween: Resurrection formed another; and Rob Zombie's Halloween (2007) and Halloween II (2009) were remakes. The 2018 film functioned as a direct sequel to the 1978 original and ignored the intervening sequels.
That continuity reset is why the count can be confusing. The 2018 film was still the franchise's 11th released feature, even though it discarded most earlier storylines. Later releases—Halloween Kills in 2021 and Halloween Ends in 2022—raised the total to 13 films.