Steam Machines are Valve's attempt to bring a simpler, console-like experience to the PC landscape. So naturally, they'll need to support some form of couch co-op like console gamers tend to want. Developers attending Valve's Dev Days this week say that it can support up to 16 controllers on one device, which could make for some mighty raucous gaming sessions.
Tweets from Zero and One Computing (via Kotaku) were among the ones talking about the limit. How to make a player count that size manageable is another matter. Though Zero and One's tweets note that split-screen in a 4x4 grid with an appropriate TV setup would result in more pixels per player than N64 launch titles, the N64 isn't exactly the watermark of modern standards.