A deck can have any number of cards named Shadowborn Apostle. {B}, Sacrifice six creatures named Shadowborn Apostle: Search your library for a Demon creature card, put it onto the battlefield, then shuffle.
2013-07-01
Shadowborn Apostle's first ability lets you ignore only the "four-of" rule. It doesn't let you ignore format legality.
2013-07-01
You don't have to sacrifice the Shadowborn Apostle whose ability you activated. It can be six other Shadowborn Apostles.
2013-07-01
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.