I made you a list of all authors of files in Images_from_Wiki_Loves_Monuments_2011_in_the_Netherlands category. Get it from /home/platonides/public_html/wlm/uploaders-nl-non-unique.txt
Just count the times a name appears. You probably want an incantation like this: sort uploaders-nl-non-unique.txt | uniq -c | sort -nr