Sleep Dart Capacity

Trust me, as you get better at the game, you'll find yourself using the sleep darts less and less. Some tips-

1) Always look for alternate ways around a guard. Can you climb above them? is there a rat you can posses?

2) Bend Time level 2 is a great power for stealth playthroughs. You can activate it and simply run past the guards, they won't see you because time has stopped.

3) Sometimes, you just have to be patient.