Preferably, all of them.
If you have fantastic data, you should be able to draw a straight line through all of the points.
However, this is not true in most cases.
When you have a scatterplot where not all of the points line up, you have to try your best to draw a line that goes through the middle of the group of points, like this:
You can find the exact line that "best fits" your points by using a graphing calculator (it should be called "linear fit").