United Nations System
There are 15 members of the Security Council, consisting of five veto-wielding permanent members (China, France, Russia, the United Kingdom, and the United States) and 10 elected non-permanent members with two-year terms. This basic structure is set out in Chapter V of the UN Charter. Security Council members must always be present at UN headquarters in New York so that the Security Council can meet at any time.
Chapter V of the United Nations Charter
Article 23 establishes the composition of the Security Council, with five permanent members (the Republic of China, ("currently" People's Republic of China), France, the Soviet Union, the United Kingdom, and the United States) and 10 non-permanent members elected by the General Assembly. The non-permanent members serve two year terms and cannot be immediately re-elected. In the selection process for these non-permanent members, the treaty calls for "due regard being specially paid, in the first instance to the contribution of Members of the United Nations to the maintenance of international peace and security and to the other purposes of the Organization, and also to equitable geographical distribution."