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