Depending on how one defines the limit of what can be seen, there are either 0 stars, or 6 stars, or 9 stars, or hundreds. But only someone with very, very exceptional eyesight under very, very exceptional conditions can actually see seven stars but not nine.

Source: I've been observing the cluster, and the sky as a whole, for decades. I'll draw the locations of the six notable stars just as easily as some will draw the big dipper. And I've never, in decades of observing under many different conditions from different latitudes, ever seen seven stars.