How many countries had to ratify the World Heritage Convention before it took effect?

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Ajanta Caves: reclining Buddha — World Heritage site Reclining Buddha statue, Ajanta Caves, north-central Maharashtra state, India.

The Convention Concerning the Protection of the World Cultural and Natural Heritage, which established the World Heritage site program, was adopted by UNESCO in 1972. It formally took effect in 1975 after being ratified by 20 countries.1