10 USC 5441 - Prescribed number; vacancies

In this chapter, the term prescribed number or number ... prescribed as applied to a grade, means the number of officers of a described corps, designation, or other category that shall be maintained in the grade concerned. Except as otherwise specifically provided, the actual number of officers in a grade may not exceed the prescribed number. Vacancies occur whenever, and to the extent that, the actual number falls below the prescribed number.