According to Islamic rulings if there’s a global caliphate every muslim in the world is required to obey that (there’s not one since 1915s). But besides that, all of the religion of Islam (rulings, practices) is capable of graceful degradation, scaling down to even just one person. If there was literally one muslim left on the face of the earth, that muslim would not lack anything in his practice of religion, virtousness or his connection with God. There’s no middleman to God like Christian (Catholic?) priests. Similarly if a group of muslims were to crash on an island in a plane accident, they’ll (and are required to) establish islamic order (sharia) there and function all by themselves. Islam is completely decentralized in this way.