You may choose a value for X that's greater than the creature's toughness. Its controller will draw that many cards.

The effect is not optional. The creature's controller can't choose to draw fewer than X cards.

If the creature is an illegal target when Nin's ability tries to resolve, it won't resolve and none of its effects will happen. No player will draw cards.

If the ability resolves, but the damage is prevented or redirected, the controller of the targeted creature will draw X cards, regardless of how much damage was dealt or what the damage was ultimately dealt to.

If the damage is lethal, the cards are drawn before the creature is destroyed (the next time state-based actions are checked).