“All seven justices voting in the majority wrote their own decisions.”
Still, you would have to say it was a 7-2 decision.
Seven justices agreed that Scott was a slave and therefore had no right to sue in court. Had Taney stopped there then Scott v. Sandford would be an obscure decision in the history of the court. But only one justice, maybe two, went further and decided that blacks were not and never could be citizens of the U.S. and that the Northwest Ordinance was unconstitutional. Taney's dicta would certainly have been challenged by the Republicans had the rebellion not gotten in the way.