It is impossible to accurately answer this question because clouds are constantly changing shape and size and are in a constant state of flux. In addition, clouds depend heavily on environmental and weather conditions which can vary drastically between different parts of the world.
However, hypothetically:
Assuming the average-sized cloud is a cumulus cloud 5 km in diameter, it would take approximately 6.7 million clouds to reach 100% sky coverage across the entire earth.
The diameter of the Earth is approximately 12,756 km. The surface area of the Earth is approximately 5.10 x 10^8 km^2
We need to calculate the area of the Earth that can be covered by 6.7 million clouds.
The area of each cumulus cloud is approximately 19.6 km^2
The total area of 6.7 million clouds would be:
6.7 million clouds x 19.6 km^2 = 131,520,000 km^2
To cover the entire surface area of the Earth, we will need a total of 131,520,000 km^2 / 5.10 x 10^8 km^2 = 2.58 x 10^-2 or 2.58% sky coverage.
Therefore, to cover the entire sky of the Earth with average-sized clouds, it would take approximately 6.7 million clouds.