The constellation Pleiades are known as the Seven Sisters, not only because there were seven of the eponymous Greek myth girls, but also because - in clear skies (ie skies from before the Industrial Revolution) - you can see seven stars twinkling next to each other.
These days of course you're lucky to see four or five. But with a telescope you can see many more than seven. So it's a pretty inaccurate name all round really.
