The Convention came into force as international law on 12 January 1951 after the minimum 20 countries became parties. At that time however, only two of the five permanent members of the UN Security Council were parties to the treaty: France and the Republic of China. The Soviet Union ratified in 1954, the United Kingdom in 1970, the People's Republic of China in 1983 (having replaced the Taiwan-based Republic of China on the UNSC in 1971), and the United States in 1988. This long delay in support for the Convention by the world's most powerful nations caused the Convention to languish for over four decades. Only in the 1990s did the international law on the crime of genocide begin to be enforced.
What was the minimum number of countries necessary to form parties? 
20