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.
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.
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.
While resolving Embereth Skyblazer's triggered ability, you can't pay multiple times to increase the bonus.
