As of my knowledge cutoff in September 2021, there are 12 films in the Halloween series. Here is a list of the films released up until that point:
1. Halloween (1978)
2. Halloween II (1981)
3. Halloween III: Season of the Witch (1982)
4. Halloween 4: The Return of Michael Myers (1988)
5. Halloween 5: The Revenge of Michael Myers (1989)
6. Halloween: The Curse of Michael Myers (1995)
7. Halloween H20: 20 Years Later (1998)
8. Halloween: Resurrection (2002)
9. Halloween (2007) - A remake/reimagining by Rob Zombie
10. Halloween II (2009) - Rob Zombie's sequel to his remake
11. Halloween (2018) - A direct sequel to the original 1978 film, ignoring the events of all the other sequels.
12. Halloween Kills (2021) - A direct sequel to the 2018 film.
Please note that the Halloween series has continued beyond my knowledge cutoff, so there may have been additional films released since September 2021.