How about Top 5% comments with the restrictions: Top 1 if less than 20 comments, top 10 if more than 200 comments (note: the values can be altered, might also be something like top 5 / 100 comments, not sure what would match this kind of challenge the best).
this could optionally be further restricted/improved by a rule like "only count comments with at least 1 vote" count, or something like "at least x% votes of top 1" (e.g. if top 1 has 50 votes, and at least 20% of the votes are needed, then only the top 5% comments with more than 10 votes are accepted)
this would scale a bit but still require a high enough participation to exclude simple bots.
I think a function of # of account followers rather than # of comments makes more sense. If tied to comments you could theoretically spam comments to allow a higher number of non-spam guesses
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you're right.
also we might restrict it further to either a question or a set of guesses (e.g. you post 1 comment with text "guesses", which can be upvoted and all the guesses are replied to it, so that either a question is top voted, or that guess collection)
that way the account value / bounty would get even bigger as well
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