You can not compare or convert units for different types of measurement. In this case you're trying to convert a unit of distance into a unit of area. It's like asking how many days go into purple. The units have to fit together to make sense.
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If they are 12 inches x 12 inches then you would need 399 tiles.
21 feet times 19 feet = 399 square feet
21*19 = 399 square feet.
The answer depends on the units used for 21x19: inches, metres, kilometres? Since that information has not been provided it is not possible to answer the question sensibly.
Circumference of circle in inches: 108*pi = 399 inches to the nearest whole number