So, finding the files/folders owned by a single user is pretty straight-forward (find ./ -user $USER), but what if you want to find out a count of the files owned by all the users in a folder tree?
# find out number of files owned by users in a folder tree
ls -lARF | cut -d ' ' -f4,4 | grep -v ':' | grep -v '^$' | sort | uniq -c | sort -rn
Basically, list all the files, then cut so you get just the username, then get rid of extraneous lines, sort together so you can get a unique count, and do a reverse numerical sort to get a descending list.
grep '.' will also work for returning characters only, also awk NF.