Alexa says to include their meta tag into site head tags. On infinite scroll pages head tags are included only once.
How does Alexa track pages with infinite scroll? Does that affects site's Alexa ranking?
The meta tags are only used to prove you're the owner of the site. It doesn't run any script to keep track of visitors or anything so as long as it's loaded once, it's ok. Alexa site ranking is based on the number of pageviews/visits from people that have the Alexa toolbar installed. An infinite scroll page will probably only count as 1 view.
Alexa rank doesn't mean anything though. If you install the toolbar and visit the site daily, you'll see the rank skyrocketing without any new visits or any real improvement.