It's said The New Mutants is the 13th and last film in the X-Men film series that began in 2000. Whether it really turns out to be the last might depend on whether it's a lucky or unlucky 13 in terms of the box office, but we can only wait and see. It might spin off a character or two into a new series.
Although a baker's dozen is an impressive number to reach, it's far from the longest-running film series or franchise.
What constitutes a film series or franchise is open to debate, of course. How much continuity or connection is needed? Some series' entries are direct continuations of a story, like the Harry Potter or Twilight movies, while others are essentially self-contained episodes with occasional nods to other films, like the Bond movies. There are, of course, other entries in the Potterverse (like the Fantastic Beasts and Where to Find Them spin-off-cum-prequel).
Do the films need to have the same central characters or simply be set in the same universe? The Marvel franchise seems largely to operate on the latter principle: characters like the Hulk and Spider-Man have their own films - and both those characters have had "reboots" with recastings - and also turn up in the Avengers films. Perhaps it is best to say the Marvel Universe is a franchise as it contains individual films and series where the characters are (usually) presumed to be in the same universe.
Does having the same character in a long line of often otherwise unconnected films constitute a series or franchise? Think, for example, of Sherlock Holmes and Dracula. While there were series starring Basil Rathbone as the former and Christopher Lee as the latter, both characters have been played hundreds of times by many actors in many unconnected films. It's not like, say, the Halloween or Friday the 13th films, spread over time but mostly connected by the same central character in each entry.
Does multimedia content help bolster a "franchise" claim? The Planet of the Apes' original film series began in 1968 and ran until 1973 with a continuing storyline, and was followed by two TV series (one live, one animated) and plenty of merchandise. In 2001 Tim Burton's "reimagining" (seems to be a fancy way of saying a looser remake of an original) of Planet of the Apes was released and about 10 years later there was a "reboot" (restarting an existing series with a new status quo).
The first Pink Panther film came out in 1963 with Peter Sellers as the bumbling Inspector Clouseau. A Shot in the Dark, the following year, was a reworking of a play into a Clouseau vehicle. To the purist, only the Sellers films directed and co-written by Blake Edwards are the true Pink Panther movies but even then there are limits. Alan Arkin starred in Inspector Clouseau (1968) with neither Sellers nor Edwards involved: they would return later. The Edwards Panther movies made after Sellers died - Trail of the Pink Panther (loaded with deleted scenes from earlier movies), Curse of the Pink Panther and Son of the Pink Panther (made in 1993, 10 years after Curse, and starring Roberto Benigni) showed Edwards needed Sellers desperately. And for some reason Steve Martin decided to take on the Clouseau character in two movies of his own.This rebooting might be part of the broader franchise, but are pointless, with a talent who should have known better than to take on another comedy legend's creation.
There have been, apparently, 36 Godzilla movies starting in 1954 - 32 Japanese, four American - do we count them as one franchise or two separate ones? And do the English-language versions of Japanese originals (like the first film, Gojira, becoming Godzilla with new scenes featuring Raymond Burr) fit in separately?
The Carry On British comedy series produced 31 movies from Carry On Sergeant (1958) to Carry On Columbus (1992). At least one a year came out for the first 20 years but after Carry On Emmanelle (1978) there was a hiatus untilthe Columbus movie, in which several of the series regulars, including Jim Dale and Peter Gilmore, appeared. Produced to coincide with the 500th anniversary of Christopher Columbus's voyage, it was a flop critically and commercially, although it grossed more than the combined take of the two more serious Columbus movies the same year.
Eon Productions' series of James Bond spy films began with Doctor No (1962) and has continued until the present, with periodic changes in the actors playing Bond (you can name your own favourite: we won't get into that here). Are the non-Eon comedy Casino Royale and Never Say Never Again, (the latter adapted from Thunderball and produced after a long rights battle) with Sean Connery returning to play 007 to be counted as part of the franchise? And did the Daniel Craig Casino Royale count as a reboot?
Star Wars has had the nine-episode "Skywalker Saga" at its centre but there have been standalone films like Solo and Rogue One (but isn't the latter really a prequel to Episode IV:A New Hope?) and the CGI The Clone Wars. And there have been numerous TV, literary and other instalments leading to a vastly expanded "universe" and quasi-theological debates on what constitutes "canon", like the notorious Star Wars Holiday Special. We won't get into that here.
Star Trek fans seem to be even more fanatical than those of Star Wars. The former is arguably as much as, if not more of, a TV phenomenon as it is a film one, so we will simply note that franchise's longevity and reach and hope the Trekkers, or Trekkies, or whatever they like to be called, live long and prosper.
For the fans of all series and franchises: discuss them, debate them, argue over them but above all, enjoy them.