First Sliver's Chosen

• WWizards of the Coast2019-06-14
  Official Ruling
  Whenever a creature you control attacks alone, count the number of instances of exalted among permanents you control. After those abilities resolve, that’s how many times the creature will get +1/+1.
• WWizards of the Coast2019-06-14
  Official Ruling
  A creature attacks alone if it’s the only creature declared as an attacker during the declare attackers step (including creatures controlled by your teammates, if applicable). For example, exalted won’t trigger if you attack with multiple creatures and all but one of them are removed from combat.

Rulings via Scryfall