How many ranks does the O-10 designation include?

The O-10 designation refers to the rank of Admiral in the United States Navy, which is an uppermost flag officer rank. In this case, there is only one rank specified under the O-10 designation, which is Fleet Admiral. The rank of Fleet Admiral is unique and does not have variations like some other ranks. This distinction makes the O-10 designation singular, confirming that there is only one specific rank classified within it. Other ranks might include various levels or positions, but for O-10, it unequivocally refers to one distinguished rank within the military hierarchy.