Is there any way to modify the columns in the File Manager of Whonix where it shows the quantity of items in each folder? All it shows is 4.0 kb for each item. 
If I remember correctly, the whonix file manager is called “Thunar” (part of xfce). So it is not a qubes-specific question. Free software like Qubes includes and borrows things from other projects so in this case you might find answers by looking it up not as part of Qubes but in general.
The XFCE docs (desktop environment) may be a good starting point: