If Congress passes a law that appoints a bunch of new supreme court justices, and that law is challenged, which justices get to decide it?

The legal question is not what you think it is. One, Can Congress do that? and How would that affect the impartiality that all sections of the Constitution on the Judiciary ASSUME 

The Justices currently on the court, of course.  The law would undoubtedly be challenged on the first appointment under the new law, so only one new justice would be involved at that point and I would fully expect that justice to recuse themselves since they would have a direct personal stake in the outcome.