Surely it is amajority of the eligible Supremes. Two of them must recuse themselves. Therefore the number of affirmative votes required is 4.
“Two of them must recuse themselves.”
The SCOTUS has no rules or requirements for the recusal of Justices.
http://www.nytimes.com/2011/12/01/opinion/the-supreme-courts-recusal-problem.html?_r=0
http://articles.latimes.com/2012/feb/12/opinion/la-oe-segall-kagan-recusal-20120212