Jace, Unraveler of Secrets

• WOTC Official Reminder Textofficial rules
  Scry — Look at the top N cards of your library, then put any number of them on the bottom of your library and the rest on top in any order.

• WWizards of the Coast2016-04-08
  Official Ruling
  If Jace’s emblem’s triggered ability doesn’t counter the first spell an opponent casts (perhaps because that spell can’t be countered), it won’t trigger again in the same turn to try to counter that player’s second spell.
• WWizards of the Coast2016-04-08
  Official Ruling
  The emblem’s triggered ability counters the first spell an opponent casts on each turn, not just that opponent’s turn.
• WWizards of the Coast2016-04-08
  Official Ruling
  If you have multiple opponents, Jace’s emblem can trigger once each turn for each opponent.

Rulings via Scryfall