It would seem that the Xbox One can support up to eight controllers at one time, according to an update to the controller’s product page.
For comparison, the Xbox 360 can support up to four controllers, while the PlayStation 3 can support seven. It is currently unknown how many the PlayStation 4 will support.
I doubt that Xbox One games will support eight-player local co-op (imagine N+ with that!), as I’m more inclined to believe that this is for situations where four people are playing, then, when they’re done, players 3 and 4 can drop out so that players 5 and 6 can jump in on the fun. Some people prefer to play with their own controllers, so this could eliminate the small issue where players are swapping out with each other and passing off controllers. Or maybe it’s not that and I’m making up problems!
The product page also notes that controllers will have a range of 30ft, which is useful if you’re say, playing online but what to go to the bathroom.