Edo Tensei sacrifices a person to bring the dead back to life using that person's body and lifeforce.
Kabuto summoned 6 at the same time to Orochimaru's 3.
That alone means he surpassed it.
If Madara is really Madara even if he is in another body Edo Tensei won't ressurect him. If Madara is dead and this guy just thinks he is Madara then well it would work.
As far as we know who ever is in the Box is something that Madara doesn't want shown.
Like say the 4th Mizukage, Orochimaru, Madara's brother its obviously someone who knows something that Madara doesn't want known. What if its Itachi's best friend....