Chapter X of the United Nations Charter
Chapter X of the United Nations Charter deals with the UN Economic and Social Council. Originally, Article 61 provided that ECOSOC would consist of 18 members, but in 1965 the Charter was amended to expand ECOSOC to 27 members, before being amended to include 54 in 1971.
Source: Wikipedia — Chapter X of the United Nations Charter (CC BY-SA 4.0)