I think you need exceptionally good eye sight to distinguish more than 6 stars. And it's not surprising that folklore stories originate around what most people's experiences are.
You don’t. You just need dark skies. Careful observers with good eyes see much more than 7.
It's also worth mentioning that, as I alluded to in another comment, myopia is much more common than it was pre-industrially, and it's still rising to what opthamalogists say is "epidemic" proportions. Good eyesight was much more common then.
