According to a technical specification published by UK retailer Play.com, Kinect is only capable of tracking two players.
The Microsoft camera sensor "tracks up to six people, including two active players," according to the spec, which Play.com notes is supplied by the manufacturer.
The term "active players" is used again when referring to Kinect's ability to track "20 joints per active player" and "map actve players to Live Avatars".
So it would seem only two people can use full-body motion control in a game simultaneously, although from what we saw of the device at E3, it could be possible for up to six to swap into and out of "active play" fairly quickly and seamlessly.
With Kinect having a horizontal field of view of 57 degrees, and depth sensor range of 1.2m to 3.5m, perhaps the restriction isn't that surprising. You'd struggle to get more than two bodies standing up in front of a TV within that area in most living rooms.
The spec doesn't say how many voices Kinect's audio sensors can recognise and track, mentioning only "speech recognition in multiple".
Kinect is due out in North America on 4th November and the rest of the world "thereafter".