Judge Barak claimed on page 4 of his opinion that 1200 civilians were killed during the 7th of October attacks, current figures show that 766 civilians were killed ,out of a total of around 1200.
Israel claims "Israeli official: Ratio of 2 civilians killed for every Hamas member "tremendously positive" in circumstances" ( 2 / 3 = 66%)
Does Judge Barak have an obligation to correct himself? Or does one of the counsels have to request it?
The ICJ Statute, art. 61 provides the rules for requesting revision:
An application for revision of a judgment may be made only when it is based upon the discovery of some fact of such a nature as to be a decisive factor, which fact was, when the judgment was given, unknown to the Court and also to the party claiming revision, always provided that such ignorance was not due to negligence.
The proceedings for revision shall be opened by a judgment of the Court expressly recording the existence of the new fact, recognizing that it has such a character as to lay the case open to revision, and declaring the application admissible on this ground.
The Court may require previous compliance with the terms of the judgment before it admits proceedings in revision.
The application for revision must be made at latest within six months of the discovery of the new fact.
No application for revision may be made after the lapse of ten years from the date of the judgment.
So, to answer your question:
As of 2021, there had been five applications for revision of ICJ judgments, and the Court had not revised any of its judgments (McIntyre, p. 480).
There is likely a power for the ICJ to correct inadvertent slips or accidental omissions. This is referred to as "rectification of error" or "rectification d'erreur matérielle." Despite the literal translation as "material error," these are not the kinds of contentious revisions that would have to go through the procedure above for revision of a judgment.
If the judge had just inadvertently included an error in the judgment (e.g. they meant to write 1200, and instead wrote 2100), where this is brought to their attention, the Court has acknowledged that it "of course [has] the power to correct, in one of its judgments, any mistakes which might be described as "erreur matérielle" (see Application for Revision and Interpretation of the Judgment of Continental Shelf (Tunis./Libya), 1985, Institution of Proceedings, pp. 30-31; Judgment, p. 198).
