If you see something too bad or too good, you can send an email to the mods: hn@ycombinator.com They usually reply soon, but it is a manual process, so use it wisely.
If you go to the [2] Algolia API, and filter "Comments in the last 24 hours". There's around 8,000 results. It's not really possible to keep track of that many comments and conversations (if he is a human that is).
My guess is that the moderation tools help him monitor for content that needs urgent attention, to keep the community and discussion healthy... so my final answer ~10% (1,000 comments per day)
[1] https://news.ycombinator.com/item?id=20643052
[2] https://hn.algolia.com/?query=&sort=byPopularity&prefix&page...
All thanks to dang and sctb anyway. :)
Good, efficient moderation requires internal tooling and view development.