In Batman: Arkham Knight, Batman can target a maximum of five enemies in one Fear Multi-Takedown.
The Fear Multi-Takedown is an upgraded stealth technique that lets Batman rapidly incapacitate several nearby opponents while remaining undetected. After each takedown, time slows briefly, giving the player a chance to select the next target before the sequence continues.
The ability is especially useful against armed guards, whose weapons make ordinary stealth approaches more dangerous. Batman must begin the attack from a suitable position and maintain the conditions needed for the move; it is not simply a combat attack that can be used against any group.
The number five is easy to confuse with the game's other multi-target mechanics, including certain gadget effects and combat abilities. The standard Fear Multi-Takedown's documented upper limit is five simultaneous targets, making 6, 4, and 3 incorrect for this question.