Leonardo, Cutting Edge

Leonardo's last ability triggers just once for each life-gaining event, whether it's 1 life from Illegitimate Business or 3 life from Guac & Marshmallow Pizza.

If Leonardo is dealt lethal damage at the same time that you gain life, he won't receive a counter from his last ability in time to save him.

If a creature spell's sneak cost was paid, the creature it becomes enters tapped and attacking the same player, planeswalker, or battle as the creature that was returned to its owner's hand to pay its sneak cost. This is a rule specific to sneak; in other cases, when a creature is put onto the battlefield attacking, that creature's controller chooses which player, planeswalker, or battle it's attacking.

If you gain an amount of life "for each" of something or "equal to the number" of something, that life is gained as one event and Leonardo's last ability triggers only once.

Spells can only be cast for their sneak costs any time you could play an instant during the declare blockers step on your turn (after your opponent has decided whether to block).