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.

Except in time of war or national emergency, not more than ten retired flag officers of the Regular Navy may be on active duty.

The President, during a war or national emergency, may suspend any provision of the preceding sections of this chapter. Such a suspension may not continue beyond September 30 of the fiscal year following that in which the war or national emergency ends.