i dont know but i think, that when sasuke saw his whole clan dead, by itachi, that itachi said that it needs 3 or 4 persons with MS. for what i dont know...
i think the statue opens an eye ech time a person activates for the 1st time his MS or something.
that statue isnt the father of madara? madara sealed him! and when he opnes all eyes he will revive. i think is madara's father (demon) that is sealed and itachi wants to revive him to fight madara, because hes the only one that can fight with madara serious. and sasuke, with the help of that can kill madara.