Popularity Rating using Bar Graphs on Itunes? What does it mean?

Apple has not stated what the bar indicates but it is a number of sales.
And some people have suggested it is relative to what selections you are viewing, not necessarily compared to the entire store/genre or whatever.

any idea as to how many times that each individual song sold? Does one black slash next to the song represent that the song has sold 10, 100, or 1000 times?

Sorry, but Apple doesn't release that information.

How did you find out that popularity rating relates to the sale of each individual song?

It can't be related to anything else since iTunes can only track sales, not what people are listening to.

Does one black slash next to the song represent that the song has sold 10, 100, or 1000 times?

I doubt it's any of those. More likely it's relative to whatever else is displayed.
If a song sold 5,000, it could have 5 bars compared with other songs but it could also have no bars if it is compared to other songs that have sold 500,000.

Again, Apple has not published any info on exactly what the rating means or how it is derived..