Multiverse

The Multiverse is the near-infinite number of different planes. Nobody really knows how many planes there truly are. Urza for one believed that there is a finite number of planes, merely more than can ever be counted in the combined lifetime of every being in existence. However another source notes that the multiverse is "boundless". Technically this is not a contradiction. As an analogy, Earth's surface does not have any bounds, but it is still finite.