I work with 2 such people (academics, prefer mercurial, use R rather than Python so I can't imagine would have had any reason to install Python 3). Lots of people use version control without being "developers", so not needing a more up to date Python (or, at just happy with Python 2).

"how many Mac/mercurial users are there?" - Hi! I'm one!

"how many of those users don't install a version of python not included with the OS". FWIW:

    % which hg
    /usr/local/bin/hg
    % ls -l /usr/local/bin/hg
    lrwxr-xr-x  1 eesmith  admin  30 May 23  2019 /usr/local/bin/hg -> ../Cellar/mercurial/5.0/bin/hg