Waterfall Aerialist

• WOTC Official Reminder Textofficial rules
  Ward — Whenever this permanent becomes the target of a spell or ability an opponent controls, counter it unless that player pays [cost].

• WWizards of the Coast2021-04-16
  Official Ruling
  If a player casts a spell that targets multiple permanents their opponent controls with ward, each of those ward abilities will trigger. If that player doesn’t pay for all of them, the spell will be countered.

Rulings via Scryfall