Click on that link for THE PLAYER.
I second this one for the sheer audacity of doing so! 
I wonder if actors appearing as themselves count?
And there’s another I had forgotten about – That’s Entertainment – which. if clips from previously made movies count, has got to be up there.
That’s what pushes The Player well into the 60s, for me (same as That’s Entertainment, FWIW).
This is what I was going to suggest, and looking at the cast list only makes me more certain this is the correct answer.
I wouldn’t include clip shows like That’s Entertainment, or any show in which the talent did not decide to be in the movie. Otherwise, you’d have to include things like Dead Men Don’t Wear Plaid. No, It’s a It’s a Mad Mad Mad Mad World has to be the top, as all of those performers and actors chose to be in that film.
I don’t know what the record is, but I’ll give a shout out to “Fast Times at Ridgemont High” and Taps for having alot of actors that eventually went on to big careers.
Some really bad films had surprisingly large casts of famous people.
The Swarm
Michael Caine
Katherine Ross
Richard Widmark
Richard Chamberlain
Olivia de Havilland
Lee Grant
Jose Ferrer
Patty Duke
Slim Pickens
Fred MacMurray
Henry Fonda
The Story of Mankind
Vincent Price (as “Mr. Scratch”, clearly The Devil, and the only good casting in the film)
Ronald Colman
Groucho Marx (as Peter Minuit)
Harpo Marx (As Sir Isaac Newton!!)
Chico Marx
Peter Lorre (As Nero)
Hedy Lamarr (as Joqan of Arc)
Virginia Mayo
Agnes Moorehead
Cedric Hardwicke
Cesar Romero
John Carradine
Dennis Hopper (as Napoleon!)
Edward Everett Horton
Francis X. Bushman (as Moses)
William Schallert (Patty Duke’s Dad on TV, here playing the Earl of Warwick)
It also featured Jim Ameche, Don’s brother, as Alexander Graham Bell. Evidently they couldn’t get Don Ameche to reprise his famous role.
Wait a minute. Have you been “Don Draper” since 2001, or did you change your username after Mad Men came out? If the former, you’ve got a hell of a knack!
Can’t forget JFK:
Kevin Costner
Sissy Spacek
Tommy Lee Jones
Gary Oldman
Jack Lemmon
Walter Matthau
Ed Asner
Joe Pesci
Wayne Knight
Vincent D’Onofrio
Jay O. Sanders
Donald Sutherland
Laurie Metcalf
Kevin Bacon
John Candy
Frank Whaley
Sally Kirkland
Brian Doyle Murray
Lolita Davidovich
John Larroquette
Ron Rifkin
Pruitt Taylor Vince
Tony Plana
Martin Sheen (narrator)
and the real Jim Garrison in a cameo, and then every important figure in U.S. politics from 1955 - 1970, in archival footage, plus Walter Cronkite.