I believe the numbers given are with the regards to the required size of the U.S. fleet, hence the number of mariners needed, to maintain the minimum number of mariners for the reserve fleet.
A recent report to Congress identified the need for 13,607 qualified mariners for sustaining the mobilization and commercial operations concurrently under highly optimistic assumptions of no loss of life or property. The identified pool of mariners will therefore provide three to four months of force projection support at best, taking into account the necessary crew rotations.