Isaiah allow himself a moment of grateful praise that he has been saved by God's mercy, and around him, he imagines a remnant of God's people similarly singing God's praise.
In his vision, that represented his call to be a prophet, Isaiah was cleansed from his sin by a coal taken from the altar. My commentaries tell me that when chapter 12 opens "in that day you will say..." it is you singular. One person is praising God for his salvation.
Isaiah himself? Maybe.
But he doesn't remain alone for long. The next time the phrase comes, in verse 4: "in that day you will say..." it is plural. Now it's Isaiah and friends. Now it's the righteous remnant, singing their praise to God.