I have a background partly in statistics and probability, so I sometimes like seeing things like this. It's fun to speculate. So, given the law of averages and established trends seen with statistical probabilities, I would surmise that the percentage of paying customers for Zorin OS would be somewhere between 2% to 5%. There is reason to believe it could be as high as 8%, though, given historical precedent for other use cases in general. But yeah. Predicting consumer behavior can be tricky; it's not an exact science, but it is an established science with a generally good track record. When it comes to paid users (in general), it can be a tough sell, especially when dealing with an audience such as Linux users.
This is part of why I have exhorted the Zorin Group to "do better" before, because I like using Zorin. I'd like for it to stick around. With as much OS-level fragmentation there is in Linux (over 500 distributions), it's tough to stand out, and make it easier for folks to pick one and stick with it. I was with DOS/Windows for over 25 years, but the way Microsoft made it back then actually made sense before they started adding a bunch of unwanted things to it like bad design, advertising, and excess bloat. So I had no reason to look elsewhere until all that started happening. Zorin OS takes me back to Windows 2000 Professional (which is not a bad thing; I liked Windows 2000 very much - perfect blend of aesthetics and function).
Anyway! There you have it. A guess of 2% to 5% who are "pro" users.