From the Document: "The Constitution provides Congress with broad powers over the Armed Forces, including the power 'To raise and support Armies' and 'To provide and maintain a Navy.' In the exercise of this authority, Congress has historically shown great interest in military recruiting, which is critical to maintaining a fully manned and capable military workforce. Congress exercises a powerful influence on recruiting through its establishment of personnel end-strength levels for the Active Components and Reserve Components. Higher end-strengths generally require a greater number of new recruits, higher rates of retention among current servicemembers, or some combination of the two. Through its oversight powers, Congress monitors the performance of the executive branch in managing the size and quality of the military workforce."
CRS In Focus, IF11147
Congressional Research Service: https://crsreports.congress.gov/