Stinging Lionfish

• WWizards of the Coast2020-01-24
  Official Ruling
  If you have more than one opponent, this ability can trigger once during each of those opponents' turns.
• WWizards of the Coast2020-01-24
  Official Ruling
  In a Two-Headed Giant game, this ability triggers no more than once during each opposing team's turn.
• WWizards of the Coast2020-01-24
  Official Ruling
  This ability triggers only on your very first spell during an opponent's turn, not the first spell after the card is on the battlefield. If you cast a spell before it's on the battlefield (including if you cast this card somehow during an opponent's turn), the ability won't trigger.
• WWizards of the Coast2020-01-24
  Official Ruling
  An ability that triggers when you cast a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

Rulings via Scryfall