The most actors playing the same role in one movie?

There's a wiki article on this too - List of characters played by multiple actors in the same film

The highest film I can find that definitely meets the requirement is:

Palindromes - eight actors playing one role:

All of the below actors play Aviva

Emani Sledge, Valerie Shusterov, Hannah Freiman, Rachel Corr, Will Denton, Sharon Wilkins, Shayna Levine and Jennifer Jason Leigh as Aviva in Palindromes (2004)

However, if we accept a slightly more unusual film:

Audition - 50 actors playing one role, 50 actresses playing the other role

This may seem like a bit of a cheat, but as you asked for movie characters, the following all play Man:

Nicholas Baroudi, Bryant Boon, Grant Boyd, Tyler Burke, Sean Buttimer, Nicholas E. Calhoun, Andrew Casanova, Steve Cavanaugh, Donovan Christie Jr., Paul Corning Jr., Adrian Corrie, Dan Cozzens, Thompson B. Crozier, Dan de Jesus, Marc T. Engberg, Jonathan Iglesias, George Iliopoulos, Benjamin Jones, Adrian Kelly, Brandon Kieffer, Aidan Kinney, Jonathan Laffey, Christopher La Forgia, Fernando L. Lamberty, Dylan Landon, Mike Lubik, Shaka Malik, Christian Manzo, Robert Maxwell, Benedict Mazurek, Adam Miller, Seth Andrew Miller, Austin Mitchell, Zachary Mooren, David Andrew Morton, Kevin Needham, Eric James O'Keefe, Philip Oros, Gabriel Pardo, Christopher Patten, Ernest G. Perry, Jeff Ronan, Andrew Saunders, David Seamon, Gianmarco Soresi, Yaron Urbas, Nicholas Urda, Chris Viemeister, Bryan Williams, and Peter Zerneck as Man in Audition (2015)

And the following all play Woman:

Szalene Anthony, Milee Bang, Christina Barrell, Caroline Bloom, Jess Brown, Julia Burnside, Erica Camarano, Blaire Carson, Senta Cassell, Teegan Curitz, Mary Davila-Delgado, Leah Dowdy, Emily Fleischer, Lolita Foster, Chelsea Frei, Kate Gunther, Casey Ann Hayward, Sarah Himadeh, Corinne Hundt, Javelyn, Emilyn Kowaleski, Erica LaTorra, Jeanne Lau, Marion Le Coguic, Mary Linehan, Cara Loften, Maribel Martinez, Ariella Mastroianni, Maggie McGuire, Velta Moore, Hadas Nuriel, Angel Pai, Cassandra Paras, Andi Potamkin, Sam Quartin, Jacqueline Real, Katelyn Sarazen, Sarah Schoofs, Alex Megan Schell, Nadia Serantes, Robin Singer, Paige Spara, Katie Rose Summerfield, Khloe Sunga, Rebecca Sussman, Amanda Tudor, Juliette Verroye, Jenna Williams, Margaret Keane Williams, and Francesca C. Zucchetti as Woman in Audition (2015)

To quote from the IMDB article on Audition:

A romance is portrayed by one hundred actors who audition for the two lead roles and the chance to perform the final terrifying act.

What is love? New York filmmaker Matt Herron spent over 15 years peeling back layers of expectations and disappointments to explore the mysteries of this complex emotion through his powerful new film, AUDITION. Set in the midst of a real-life acting competition, this cautionary narrative within a documentary is a sensually-riveting journey every adult should see.

So technically all of these people are playing the same role - that of the "man" or that of the "woman". If you check the film's credits, they are all credited as so, and as the OP asked Which movie-character was played by the most amount of distinct actors in a single movie?, this does meet the requirements.

Depending on your definition of "Played" and "Movie", Doctor Who counts.

First is the "Doctor Who: The Curse of Fatal Death" comedy special, (a spoof of DW prior to the revival, really), we have 5 doctors in as many minutes:

• Rowan Atkinson (The 9th Doctor)
• Richard E. Grant (The Quite Handsome 10th Doctor)
• Jim Broadbent (The Shy 11th Doctor)
• Hugh Grant (The Handsome 12th Doctor)
• Joanna Lumley (The Female 13th Doctor)

No actual group shot exists though:

The Classic Who special "The Five Doctors" had... Five actual Doctors: - Peter Davison (the Fifth Doctor) - Patrick Troughton (the Second Doctor) - Jon Pertwee (the Third Doctor) - Ricard Hurndall standing in for the Late William Hartnell (The First Doctor) - Tom Baker (the Fourth Doctor, mostly stock footage)

The New Who regular 7th season finale "The Name of the Doctor" had all 11 standard Doctors, plus John Hurt as the War Doctor, while the special "The Day of the Doctor" had those 12 plus the following 12th Doctor, Peter Capaldi, for a total of 13 actors playing one character in a single movie (14 if you count Eccleston's stock footage and his stand in at the end as two different actors). Of course, most of the Doctors were stock footage, but at the end they had multiple stand in or cgi versions standing together.

CW turned answer originally posted as comment by Chanandler Bong

In I'm Not There Christian Bale, Cate Blanchett, Marcus Carl Franklin, Richard Gere, Heath Ledger, and Ben Whishaw (6) all play characters based on Bob Dylan.

In The Imaginarium of Doctor Parnassus Heath Ledger, Johnny Depp, Colin Farrell and Jude Law (4) all play Tony.

"Lessons", the first episode of the last season of Buffy the Vampire Slayer, ends with a scene in which the First Evil serially impersonates seven characters, and thus is portrayed by seven actors (as well as at least three others in other episodes).

Does the actor have to be human? If not, then it might be Marley & Me with 22

Because the film covers 14 years in the life of the dog, 22 different yellow labradors played the part of Marley (as revealed in the special feature Finding Marley on the DVD).

As Mr_Thyroid pointed out, this is beaten by Babe with 48

Since Anton brought up impersonations that visually perfectly mimic the original character I also consider the T-1000 from Terminator 2 valid, and from this list the following impersonations can be extracted:

• Robert Patrick as the T-1000 posing as a Cop (but as @cde mentioned, not the one investigating its appearance)
• Jenette Goldstein as John Connor's foster mother Janelle Voight
• "Lewis" (the hospital warden, who was actually played by twins)
• Linda Hamilton and her twin sister Leslie as Sarah Connor, to my knowledge the only one where the original survived
• (possibly) the Cop Biker

That's four to six actors depending on how you count the twins, so let's say 5

In The Curious Case of Benjamin Button, seven people portray the character of Benjamin Button. Here is a list of actors who portray Benjamin:

• Brad Pitt as Benjamin Button (adult), Caroline's biological father
  • Robert Towers as Benjamin Button (apparent adult)
  • Peter Donald Badalamenti II as Benjamin Button (apparent adult)
  • Tom Everett as Benjamin Button (apparent adult)
  • Spencer Daniels as Benjamin Button (apparent age 12)
  • Chandler Canterbury as Benjamin Button (apparent age 8)
  • Charles Henry Wyson as Benjamin Button (apparent age 6)

_{The Curious Case of Benjamin Button}

In The Good Place episode "Janet(s)", Eleanor Shellstrop has an identity crisis while in a reality-warping part of the afterlife, and as a result gets played by a large number of different actors. IMDB lists 10 of them, but I think that's only the ones who have lines; in this clip, I count somewhere in the area of 30 different people (most only for a second or two).

Casino Royale (1967) should at least get an honorable mention. About half the characters on screen were named "James Bond (007)". At least 6, probably several more.