How many member countries did the UNO had on its formation in 1945 ?

There were 51 Founding Members in 1945. In accordance with Article 110, the Charter entered into force on 24 October 1945, after ratification by the five permanent members of the Security Council and a majority of the other countries.
The Charter was signed on 26 June 1945 by the representatives of the 50 countries. Poland, which was not represented at the Conference, signed it later and became one of the original 51 Member States.
The United Nations officially came into existence on 24 October 1945, when the Charter had been ratified by China, France, the Soviet Union, the United Kingdom, the United States and by a majority of other signatories. United Nations Day is celebrated on 24 October each year