20 (19½ actually)
You would need enough fabric to make a circular cloth ten feet in diameter (assuming you mean the tabletop is a circle eight feet in diameter - you aren't exactly clear on that - whether the tabletop is 8 feet in diameter or 8 feet in circumference).
The area of a circle with a diameter of 8 feet is: 50.27 square feet.
A circle with a diameter of 6 feet has an area of 28.27 square feet.
The area of a circle, if the diameter is 4.3 feet, is: 14.52 square feet.
The area of a circle that has a diameter of four feet is: about 12.57 square feet.
The area of a circle, if it has a diameter of 13 feet, is: 132.7 square feet.
The area of a circle with a diameter of 2 feet is 3.14159 square feet, approximately.
The surface area of a sphere with a diameter of 18 feet is: 1,020 square feet.
A circle that has an area of 78.5 square feet has a diameter of 10 feet.
The diameter is 4.222 feet.
A circle with a diameter of 66 feet has an area of 3,421.19 square feet.
The area is 50.27 square feet.