Adinnieken on 31 July 2013
Contrary to published reports comparing the Xbox One's friends list versus the PS4's friends list, the Xbox One has a virtually unlimited friend list. In an interview with Angry Joe shortly after Microsoft's E3 conference, Larry Hyrb (Xbox LIVE's MajorNelson) clarified Xbox One's friends list.
http://www.youtube.com/watch?v=6RtSGFryKwo#t=2m26s
While there isn't a hard limit given, it sounds as though the Xbox One's friends list can expand as much as you might possibly and practically need.
