Final answer:
The National Security Act of 1947 established the Marine Corps as one of the branches of the Department of Defense, but it does not specify the number of missions. The Marine Corps has multiple missions, such as amphibious operations and expeditionary warfare.
Step-by-step explanation:
The National Security Act of 1947 reorganized the U.S. military to contain Soviet expansion. It established the Department of Defense and created the Marine Corps as one of its branches. However, the act does not specify the exact number of missions for the Marine Corps. The Marine Corps has multiple missions, including amphibious operations, expeditionary warfare, and conducting joint operations with other branches of the military.