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If you control multiple lands with the same name, only one of those lands will count toward the seven or more required to create a Zombie. For example, if you control four lands named Plains, two named Island, and one named Field of the Dead, you control three lands with different names.

Field of the Dead's last ability counts itself and the land entering the battlefield in addition to whichever other lands you control.

If multiple lands enter the battlefield simultaneously, possibly including Field of the Dead itself, all of those lands are counted. For example, if you sacrifice eight lands while resolving Scapeshift and search your library for five different basic land cards, two Field of the Dead cards, and one other land card with a different name, you'll create sixteen Zombie tokens.