This could help a lot of movies. There is something to choosing actors who don’t look similar to each other and dress much the same… The Prestige drives me bats because the two primary antagonists played by Bale and Jackman look so much alike in the movie and I’m not the best at facial recognition. First time I saw L.A. Confidential it was on one of those small old fashioned TVs that we used in the previous century and all the men sporting exciting 50s style black on white clothes and short haircuts looked pretty much alike.
I count 23 actors playing themselves in This Is the End.
The Wikipedia entry has been updated again. There have been twelve movie versions of this. Three were American (although one of those somehow avoided mentioning in the credits where the story came from even though it’s not just a similar plot but an almost identical one with names changed), one was British, two were German, two were Japanese, three were Indian, and one was Iranian. Surprisingly, this isn’t a record for how many remakes there have been of a movie. There’s this one:
If the question is what book has the most film versions of it, perhaps the answer is Alice’s Adventures in Wonderland by Lewis Carroll:
There are 208 films somehow related to Carroll’s works already in existence and another 15 in production, most of which are related to Alice’s Adventures in Wonderland. It’s hard to tell how many of them are fairly close to the book though. Many of them only slightly refer to the book.
In Necessary Roughness the coach decides they need to scrimmage against another team. They get a team from the local state prison to come play. The prison team has Dick Butkus, Earl Campbell, Roger Craig, Ben Davidson, Tony Dorsett, Evander Holyfield, Ed ‘Too Tall’ Jones, Jim Kelly, Jerry Rice, Herschel Walker and Randy White.
If you’ve ever wanted to see Kevin Bacon in a pink negligee, that’s where you can.
On an only slightly more serious note, the “Sharknado” movies are full of them. My favorite were ZZ Top in a crowded subway.
There’s a 25th remake of the 2016 film Perfect Strangers currently in production, according to the IMDb entry on it:
The worse case of something like that for me was the 1998 film The Thin Red Line - also, incidentally, a movie with a lot of famous cameos. The problem was, while there were plenty of familiar faces in the movie, arguably the two main roles were played by then-unknown actors, Jim Caviezel and Adrian Brody, who at the time looked very much like each other. In fact, they resembled each other so much that until the very end of the movie, I had no idea that they were two separate characters. They looked the same, they dressed the same, they spoke (and thought - half the dialogue in the film was internal monologues) in the same emotionless monotone, and they had no scenes together. I think my error was perfectly understandable
I recently saw Naked Gun 33 1/3 and it had many cameos. It looks like around 15 or so.
Shocked that It’s a Mad Mad Mad Mad World hasn’t been mentioned yet. Is Carl Reiner’s appearance a cameo, or a part with very short screentime?
EDIT: Search fails me again. 
Oh, durned commas…
What_Exit listed it in post #2.
You must have somehow seen an early version, because Brody was supposed to be one of the main characters, but director Terrence Malick cut it down to a minor role.
Adrien Brody’s Small Part In The Thin Red Line Started Out As A Lead
I have no way of knowing how much screen time Brody and Caviezel got, respectively, because as I said, I thought they were the same person.
The Wikipedia entry now says that there have been thirteen versions of this. Three are German, one is British, three are American, two are Japanese, three are Indian, and one is Iranian. Two of those film versions are animated. There is also a British stage musical version. Some of these have apparently gotten away with using the same plot and not paying royalties.
Also, from the previous year, Stage Door Canteen, which featured a lot of actors with stage experience in cameos (not too many who were exclusively stage stars, or the movie would have had limited appeal).
These were actual places, though, where real stars and actors worked for free to give all military people, enlisted and officers, some memorable R&R. I’m pretty sure all the cameos in the movies were unpaid as well.