Sixteen people tops per certain maps if you play online. Through one local system, four.
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Call of Duty World at War, Modern Warfare, Modern Warfare 2, Black Ops, and Modern Warfare 3 all have online multiplayer.
Infinity ward has not developed a way for 2 people 2 go online for any console, you can play split screen together but online no
No.
Play online
You can't, modern warfare 2 has only one player online but it has splitscreen which is only local play.