It would count as two impressions, because that person viewed 2 pages. However this assumes that the visitor stayed on the page for a certain period of time. I don't know how long that period is, I assume it is not very long, maybe 1 minute? Maybe even 30s. I suspect that if somebody arrives on a page and hits the "back button" immediately, that doesn't count as an impression for payment purposes.
The other thing is that people are increasingly use ad blockers on the internet now. A visitor who uses an ad blocker will not count as an impression, because they will not see the ads.
Also remember that you share the views with HP. So basically impressions (in the HP ads program) are 60% of your views, where the visitors stayed on the page for a certain amount of time, and didn't use ad blockers.