Embereth Skyblazer
Creature — Human Knight
During your turn, this creature has flying.
Whenever this creature attacks, you may pay 
. If you do, creatures you control get +X/+0 until end of turn, where X is the number of opponents you have.
"We fight with valor in our hearts and fire in our veins!"
Daarken
Throne of Eldraine (ELD)
• #321Rulings
The value of X is determined only as Embereth Skyblazer's triggered ability resolves. It doesn't change later in the turn if opponents leave the game.
(wotc • 2019-10-04)
While resolving Embereth Skyblazer's triggered ability, you can't pay {2}{R} multiple times to increase the bonus.
(wotc • 2019-10-04)
In a multiplayer game, opponents who have left the game before the ability resolves aren't counted by Embereth Skyblazer's triggered ability.
(wotc • 2019-10-04)
Rulings
The value of X is determined only as Embereth Skyblazer's triggered ability resolves. It doesn't change later in the turn if opponents leave the game.
(wotc • 2019-10-04)
While resolving Embereth Skyblazer's triggered ability, you can't pay {2}{R} multiple times to increase the bonus.
(wotc • 2019-10-04)
In a multiplayer game, opponents who have left the game before the ability resolves aren't counted by Embereth Skyblazer's triggered ability.
(wotc • 2019-10-04)