Asking a human to research 20 witnesses is doable but stretches the limits. Asking a human to research 100 witnesses is a fool's task and cannot work well in practice.
I could probably say why I vote for about half of the 30 witnesses I vote for. The other half I don’t even remember why. Maybe they impressed me a few years ago and I’ve since forgotten my reasons.
that would be most people. even 15 is a lot, but I think 20 is the most we can ask in practice and get good results.