According to the question

Radius of each spherical lead shot $=r=4.2/2=2.1cm$

The dimensions of the rectangular lead piece $=66cm\times 42cm\times 21cm$

So, the volume of a spherical lead shot $=4/3\pi {{r}^{3}}$

$=4/3\times 22/7\times {{2.1}^{3}}$

Then, the volume of the rectangular lead piece $=66\times 42\times 21$

The number of spherical lead shots $=$ Volume of rectangular lead piece/ Volume of a spherical lead shot

$=66\times 42\times 21\left( 4/3\times 22/7\times {{2.1}^{3}} \right)$

$=1500$