A Google search doesn't come up with a definitive answer so I want to see if we can collectively come up with the cast with the most Oscar winners.
My first guess would be The Dark Knight Rises, but I doubt it has the most.
Christian Bale (Supporting Actor, The Fighter)
Morgan Freeman (Supporting Actor, Million Dollar Baby)
Michael Caine (Supporting Actor, Hannah and Her Sisters and The Cider House Rules)
Anne Hathaway (Supporting Actress, Les Miserables)
Gary Oldman (Lead Actor, Darkest Hour)
Marion Cotillard (Lead Actress, La Vie en Rose)
So that's 6, but there are surely more that top it.
EDIT: The winner as of now is u/schnit123, coming up with a very clever answer: the documentary Side by Side which interviewed 16 Oscar winners. The definition of "cast" was not made clear in the post, so I'll take it!
EDIT2: Looks like u/lebvidox has second place currently with 1992's The Player with 13 winners. Even though most of those are short cameos, I've been convinced that they count. Way to go!
EDIT3: u/SoCalRocket has the third place film so far: 1996's Hamlet with 9 winners.
EDIT4: u/IVChioco is tied for third: How the West Was Won with 9 winners as well. Very nice!
EDIT5: Another submission from u/lebdivox has made the list with 9 winners also: (EDIT: Correction) 1965's George Stevens religious epic The Greatest Story Ever Told. Great job guys, keep them coming!