21. The formula for the volume of a sphere is $V=\frac{4}{3} \pi r^{3}$, where $r$ is the radius of the sphere. In a cylindrical container, several solid iron spheres with the same radius as the cylinder's base can be placed. When water is poured into the container, the volume of water is six times the volume of one iron sphere when the water level just reaches the top of the container. The number of iron spheres in the container is $\qquad$.
$12$