Approximately 25.
Of the 5 grades of sand used in the United states, medium sand has particles ranging from 0.25 mm to 0.5 mm. For convenience, I'll use 0.5mm as the diameter of a grain of sand. Human cells range from a volume of 30 cubic micrometers (human sperm cell) up to 4 million micrometers (human egg cell). For the average human cell, I'll use an osteoblast with a volume of 4000 cubic micrometers or a diameter of 2x10^-5 meters.
Now just take the diameter of a grain of sand and divide by the diameter of a human cell.
5 x 10^-4 / 2x10^-5 = 5/2 x 10^(-4 - -5) = 2.5x10^1 = 25
So the diameter of a grain of sand at 0.5 mm is approximately 25 times larger than that of a human cell at 0.02mm
