Infinity Ward: Two-player the 'magic number' for co-op

"More than that and Modern Warfare 2 would become a 'clusterf**k'."
Two-player is the “magic number” for co-op, Call of Duty: Modern Warfare developer Infinity Ward has said.
The last Call of Duty game, Treyarch's World at War, featured four-player campaign co-op and a bonus four-player survival Zombie Mode.
Call of Duty: Modern Warfare 2, due out worldwide on November 10, won't have a campaign co-op mode at all. Instead it will have a two-player Special Ops mode, “designed to be replayed over and over again”.

Read more at videogamer.com