4, if you base the answer on PHB rules. You can only use Sentinel to take away movement on an opportunity attack. Opportunity attacks are reactions. You are only allowed 2 reactions per round. With the Swing Attack maneuver, you can attack 2 opponents with 1 attack. If both your reactions use the Swing Attack maneuver, you can attack 4 opponents with 2 reactions. If you hit all 4, you can use the Sentinel feat to reduce their movement to zero, all within the same round.