NEW YORK – As this summer’s slate of sequels marches on, Hollywood is planning even more installments of their biggest franchises. With an indefinite number of future “Spider-Man” movies on tap and an 11th “Star Trek” coming next year, how many sequels are enough?

“The marketplace decides,” says Tom Ortenberg, president of theatrical films at Lionsgate, which will release “Saw IV” in October and bring back Sylvester Stallone next year in “John Rambo.” “If the marketplace is there to support it, you find a way to get it done.”

“In such a crowded marketplace, being able to break through the clutter is half the battle,” he says. “Sequels, already branded, make it that much easier to break through that clutter with a title that is already familiar to consumers.”

By the end of August, 14 sequels will have passed through movie theaters, many of them exiting with bags of money in tow. “Spider-Man 3” has grossed more than $335 million, “Shrek the Third” $318 million and “Pirates of the Caribbean: At World’s End” $304 million. All three have combined for over $1.5 billion overseas.

Since the days of Aristotle, the number 3 has been considered the most perfect figure in dramatic arts. But studios still find it hard to decline potential fortunes for the sake of a trilogy’s neat narrative structure.

A fourth “Shrek” film is scheduled for 2010. Jeffrey Katzenberg, chief executive of DreamWorks Animation SKG Inc., recently said a fifth will follow.

There are no current plans for another “Pirates” film, but Johnny Depp has indicated he’s open to it. Sony Pictures and Marvel Studios have said with certainty that “Spider-Man” will be back, but writer-director Sam Raimi and stars Tobey Maguire and Kirsten Dunst have voiced reluctance to continue.

“I know there’ll be a `Spider-Man 4, 5 and 6,'” Raimi said earlier this year. “I just won’t be the guy that’ll probably write the story, because I’ve got to clear my head.”

Instead, Raimi has been linked to “The Hobbit,” a film based on J.R.R. Tolkien’s prelude to the “Lord of the Rings” series. Peter Jackson, who directed the well-planned “Lord of the Rings” trilogy, is suing New Line Cinema over profits from “Rings,” complicating plans for “The Hobbit.”

It wasn’t always like this.

“`Sequel’ was a dirty word,” says Paul Dergarabedian, president of the box-office tracker Media By Numbers. “They were seen as an exploitive way to capitalize on a brand name recognition of a popular movie by making the same movie and putting a `2′ behind it.”

Dergarabedian recalls feeling a shift when 1999’s “Austin Powers: The Spy Who Shagged Me” made more money in its first weekend than the original made in its entire theatrical run. Now, sequels don’t ride the coattails of the original so much as they gain momentum, swelling into larger and larger behemoths with each progressive installment.

But how long will audiences keep paying to see the same franchise? Part of the trick is to make the old seem new again.

“John Rambo” carefully avoids what would be an unappealing “IV.” Stallone did the same earlier this year with “Rocky Balboa,” the sixth “Rocky” film.

Both are part of what might be called nostalgic sequels. Included in the trend is the recently released “Live Free or Die Hard,” the fourth installment in the “Die Hard” series, a dozen years after its last. There’s even a planned sequel to “Alien vs. Predator,” which itself was based on old franchises. “Alien vs. Predator 2” is scheduled for release this Christmas.

The most anticipated of nostalgic sequels is the as-yet-titled fourth “Indiana Jones.” Currently shooting, the new “Indiana Jones” struggled for years to line up everything before beginning production with Harrison Ford starring and Steven Spielberg directing. Sean Connery declined to participate.

There have even been rumblings of a fourth “Ocean’s” movie following this summer’s “Ocean’s Thirteen.” Opening Aug. 3 is “The Bourne Ultimatum,” and while it’s based on Robert Ludlum’s third and final “Bourne” novel, author Eric Van Lustbader recently picked up the series, writing two more “Bourne” books.

A spokesman for NBC Universal said that “Ultimatum” concludes the questions posed by the first chapter of Jason Bourne’s adventures, “with the potential for continuation remaining open.”

“Transformers,” which was based on the `80s toys and animated TV show, has grossed over $223 million in two weeks and seems destined for at least one sequel. Martin Levy, a marketing executive at DreamWorks (which co-produced the movie with Paramount), says: “It’s a very logical thing to think that we can look forward to another one.”

Whether the source material is serialized or not is an important part of the sequel craze. “Harry Potter” came tailor-made for a seven-part franchise, with a film based on each book. The fifth movie, “Harry Potter and the Order of the Phoenix,” has made over $139.7 million since opening last week.

Kevin Feige, president of production at Marvel Studios, believes the recent popularity of sequels is because many are based on “ongoing sagas.” Marvel, which holds the rights to many of the most popular comic book characters, is well situated to draw from its extensive library for franchise sequels.

Feige says another “X-Men” film is possible and that spin-offs for Wolverine and Magneto are in the works. A third “Fantastic Four” following this summer’s “Rise of the Silver Surfer” edition is also possible, and Feige says a Silver Surfer stand-alone script has been “moved to the forefront” of discussions.

“There’s always stuff that ends up in our back pocket or that we put to the side in our if-we-should-be-so-lucky-next-time folder,” says Feige of Marvel’s comic book source material. “It’s not difficult finding which storylines or characters we’re going to use in one of our films. It’s difficult deciding which ones not to use.”

In this way, “Spider-Man,” “Harry Potter” and “Batman” (the seventh of which will be released next year) have more in common with a serial like the James Bond films (based on Ian Fleming’s novels) than “Police Academy.”

Peter Bart, editor-in-chief of the daily Hollywood trade paper Variety, credits the studios for improving the quality of their franchise sequels.

“The studios have learned how to win at this business,” says Bart. “They’ve picked the right subjects and understood how to exploit them internationally.”

But as the sequel numbers pile up, Bart says, the studios are moving into “new waters.” Every time they go back to the well, the riskier it gets to depend on the same franchises.

“The big question,” he says,” is how long will they endure?”

A service of the Associated Press(AP)