The AB Duke generally has a yield of +/- 15 students per year. I believe around 23 scholarships are offered, with a, projected yield of about 2/3. In 2013 the yield was unusually low (8 accepted, so around a 33% yield). But in 2014 19 were accepted. I believe that the 2010, 2011 and 2012 classes each had 15, so the 8 does seem to be an anomaly.
http://article.wn.com/view/2014/05/07/Nineteen_Selected_for_AB_Duke_Scholarships_Duke_University/
https://today.duke.edu/2012/05/abdukes2012
https://today.duke.edu/2011/05/abduke11
https://today.duke.edu/2010/05/abduke.html