How to list commiters sorted by number of commits (commit count)?

There is no pure Mercurial solution, but you can do something like:

hg log --template "{author|person}\n" | sort | uniq -c | sort -nr

If you want to be able to type hg shortlog, you can add the following to your .hgrc or mercurial.ini:

[alias]
shortlog = !hg log --template "{author|person}\n" | sort | uniq -c | sort -nr