10 USC 12001 - Authorized strengths: reserve components

(a) Whenever the authorized strength of a reserve component (other than the Coast Guard Reserve) is not prescribed by law, it shall be prescribed by the President.
(b) Subject to the authorized strength of the reserve component concerned, the authorized strength of each reserve component (other than the Coast Guard Reserve) in members in each grade is that which the Secretary concerned determines to be necessary to provide for mobilization requirements. The Secretary shall review these determinations at least once each year and revise them if he considers it necessary. However, a member of the reserve component concerned may not, as a result of such a determination, be reduced in the members reserve grade without the members consent.