Okay so I found a couple articles saying it's 1000 but I don't know if it's the truth.
http://news.softpedia.com/news/Xbox-One-Has-1-000-Xbox-Live-Friends-Limit-List-of-Buddies-Is-Shared-with-Xbox-360-361535.shtml
http://www.engadget.com/2013/05/21/xbox-one-installs-friend-list/
http://www.digitalspy.ca/gaming/news/a483495/xbox-one-to-expand-friend-lists-to-1000-friends.html
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