What movie has the highest number of famous actors and actresses starring in it?

George Clooney as Danny Ocean
Brad Pitt as Rusty Ryan
Matt Damon as Linus Caldwell/Lenny Pepperidge
Bernie Mac as Frank Catton
Elliott Gould as Reuben Tishkoff
Casey Affleck as Virgil Malloy
Scott Caan as Turk Malloy
Eddie Jemison as Livingston Dell
Don Cheadle as Basher Tarr/Fender Roads
Shaobo Qin as "The Amazing" Yen/Mr. Weng
Carl Reiner as Saul Bloom/Kensington Chubb
Andy Garcia as Terry Benedict
Eddie Izzard
Ellen Barkin as Abigail Sponder
Al Pacino as Willy Bank
Vincent Cassel as François Toulour
Bob Einstein as FBI Agent Robert "Bobby" Caldwell
Olga Sosnovska as Debbie
David Paymer as The VUP (The real Five Diamond Award reviewer.)
Julian Sands as Greco Montgomery
Angel Oquendo as Guard Ortega
Jerry Weintraub as Denny Shields
Scott L. Schwartz as "The Bruiser" (seen cashing a large amount of casino chips)
John K. Peccer as Jesse Ghardi
Oprah Winfrey as Herself
Noureen DeWulf
This is the ensemble cast for "Oceans 13"
https://en.wikipedia.org/wiki/Ocean's_Thirteen

Since fame is an arbitrary thing, this is hard to answer, but "The Longest Day" and "Around the World in Eighty Days", each with about 50 noted performers, come to mind.
http://www.imdb.com/title/tt0048960/
http://www.imdb.com/title/tt0056197/

"Hollywood Canteen" also has dozens of stars, though since they play themselves mostly it's not quite fair methinks.
http://www.imdb.com/title/tt0036922/

"Forever and a Day" had a large number of British stars make an appearance.
http://www.imdb.com/title/tt0035897/fullcredits#cast
"It's a Mad Mad Mad Mad World" had numerous famous comedy stars in it. http://www.imdb.com/title/tt0057193/fullcredits

Oh and take any form of transport in the 1970's, or large building, or disaster-prone city, or location attractive to killer bees, and you had big stars all lining up to be filmed dying in it, in more ways than one, usually in widescreen panavision and sometimes with the benefit of Sensurround.
http://www.imdb.com/name/nm0000740/
It might be a coincidence, but they all got really big paychecks to do so.

"That's Entertainment" is a who's-who of classic Hollywood.

http://www.imdb.com/title/tt0072272/