No single headcount can adequately express every condition needed for tawātur. The independence of those speaking, their access to the events, their mutual connections, and the wider circumstances affect whether the report yields certainty.
A large number of people might all trace a claim to one original source. A smaller collection of genuinely independent observations may have a very different evidential character. Thus treating ten, forty, or seventy as a universal mechanical threshold misses the point of the discussion.
The central criterion is whether the actual pattern of reporting excludes plausible collective fabrication or other shared sources of error, given the type of fact reported and the conditions under which it circulated.