A deck can have any number of cards named Shadowborn Apostle.
, Sacrifice six creatures named Shadowborn Apostle: Search your library for a Demon creature card and put it onto the battlefield. Then shuffle your library.
7/1/2013 Shadowborn Apostle’s first ability lets you ignore only the “four-of” rule. It doesn’t let you ignore format legality.
7/1/2013 You sacrifice the six creatures when you activate the ability. You can’t respond to that ability by activating the ability of one of the other Shadowborn Apostles you just sacrificed.
7/1/2013 You don’t have to sacrifice the Shadowborn Apostle whose ability you activated. It can be six other Shadowborn Apostles.