Let me walk you through the steps that are the default in the industry:

1.) A steam account (or PS or xbox account) purchases a copy of a game.

2.) If that account at any time fulfills the condition for an achievement (like finishing chapter 1), that account sets a flag that it has reached that milestone.

3.) you divide the amount of flags by the number of copies sold.

That is it. That is how you correlate copies sold to the chapters that are finished. While it does not you an idea how many people finished the chapters, you do know how many copies have reached a certain progress.

You could write a custom reporting tool that tells rockstar when a new campaign starts and how it is going, but there simply isn't a reason to assume rockstar did this, especially since that would require you to be always online.