Hi Mike,
It's difficult to relate units shipped to developer community, as any graph of "units shipped per developer" would look like an exponential "long tail" - there are some very high volume products built with ESP-IDF, where the development team (even if it's fairly large team) is very small if measured as a ratio of the number of units shipped. We can't provide details due to customer privacy, commercial agreements, etc.
Then there is a long tail starting with medium and small companies with more modest shipping volumes, down to hobbyists and folks building one-off things where there might only be one or two actual ESP32s involved. Due to the statistics of such a "long tail", these are the folks mostly commonly seen here on the forums, GitHub, etc (because there are simply more of them in total).
As an aside, something I personally feel good about is that on these forums and GitHub it's possible to talk to the real ESP-IDF developers, without first having to show you're planning production with X many units. I know it's not perfect and not every question gets an answer from an Espressif employee, but it's something we care about. And of course this community has a bunch of other folks who know a ton about ESP32, as well!
As another developer-focused metric, the
ESP-IDF github project currently has 6.3k stars and 4k forks. You could compare this to other open source software frameworks on GitHub to get some general idea how many developers are looking at them, relatively speaking.
EDIT: I'm curious if you can share more about your motivation for this. Is it: "if I build my product on ESP32 will I be able to get help?", "can I hire a developer who can work with ESP-IDF?", "if I skill up on ESP-IDF will this help my career?", or something different?