Let's set a limit of a maximum of 10 in the cast. Total. Just for discussion purposes.
Some that come to mind are My Dinner with Andre, The Quiet Earth, Sleuth (with Caine and Olivia, of course), Dark Star (although sometimes this is listed as having 11 in the cast) and Alien.
Often taking places in cramped quarters (spaceships, lifeboats, or shipwrecked on islands) some of these movies provide some of the more memorable performances and scenes of all movies - and it ain't just special effects, ma!
Sleuth still does it for me. After all these years, Caine and Olivia just seem to match each other as if they're really going for it, not just playing their parts in this contrived plot. Wonderful!