Memoricide instructs its controller to search for ‘any number’ of the named card in the hand, graveyard and library of the targeted player. ‘Any number’ can mean zero. This means that is is possible for a player to miss, intentionally or not, a copy of the named card in a graveyard. This is a bit different from other cards where you search a public zone, like a graveyard, for all copies of an object.