Data for Alexa's reach, rank, and search analytics are collected from millions of users who have the Alexa toolbar or extension installed. This enables Alexa to collect, anonymize, and track web traffic over time.

It’s important to note that, according to Alexa, multiple page views of the same page, made by the same user, and on the same day, are only counted once.

Using Alexa data, and dividing Page Views Per Million Users by Reach Per Million Users, you can estimate Page Views per Visitor. Note that using Google Analytics data will give a more accurate result.