number of RNA pol II molecules in a given human nucleus?
it is highly variable even in a single cell type.

Dont know for a human cell but this is the range for ecoli: 1500-11400

check this site out. got this number from there (its quite useful for questions like these):

http://bionumbers.hms.harvard.edu/bionumber.aspx?&amp;id=101440&amp;ver=3&amp;trm=RNA%20polymerase

You can estimate protein copy numbers and there are techniques available for doing so. One of such techniques:

http://www.mcponline.org/content/11/3/O111.009613.long

You can also calculate concentration using traditional techniques like western blot and elisa and calculate approximate copy-numbers (you have to also calculate average cell volume and number of cells for protein extraction.