Currently only 6 players still have a chance at the Championship Riven skin...
http://pickem.na.lolesports.com/en-US#leaderboards/everyone
Last year, only 1 out of over 2.6 million players managed to perfectly predict the results of worlds until the end. I don't know what the standing was in the group stage, but I think the current pool is so low because not many expected Albus NoX Luna to do so well.
For those of us who already have enough points for the icon, but can't get the perfect pick skin, let's all root for Albus NoX to keep breaking people's expectations :)