I'm not at all a prescriptive grammarian so I would like to know the argument one would make for preferring the quoted title formation over the more common,

I've told you at least ten times...

Or more generally I'm asking why a grammarian bothered by things like "10 items or less" would argue for "fewer" with count nouns when the meaning of the sentence is not ambiguous.

Is "I've told you at fewest ten times..." the most correct construction, and does it matter?