Question 883942
The quickest way to calculate is to divide the volume of one sphere into the volume of the drum. 
{{{N=(pi*(D/2)^2*L)/(1.38)=(pi(21/2)(29))/1.38=7278}}}
Although quick it is inherently incorrect and overestimates the number because it doesn't account for gaps in between the spheres which normally exist. 
If you think of how cannonballs are stacked and do something similar with these spheres, the maximum density you can achieve is {{{pi/(3sqrt(2))*V[max]}}} or {{{0.741*Vmax}}}
So the maximum number would decrease to {{{5389}}}.
For more information, look up close packing or center packing of spheres in your favorite search engine.