10 U.S.C. § 8101

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.