I think I managed to get accurate results like this:
- Go to the
Review
ribbon - In the
Tracking
section, open the dropdownShow markup
- Deselect everything except comments
- Open dropdown
Show markup
again - Under
Specific people
, mark only one person - Click on
Reviewing Pane
In the reviewing pane, it now shows xx Revisions
. As far as I could see, these are all comments now, so we have the number of comments for one author.
From now on it is much easier to do the same for other authors, because you just need to change the person under Specific people
.