If you work with development tools, I am pretty sure there is still a large percentage of developers who use Windows as their primary development box. You are not looking for developers targeting Windows OS, but you are looking for how many developers are using Windows OS.
Wrong, I'm looking for people targeting Windows, not just developing on it. They can use either Windows or Linux for the development, it's the target that's interesting in this case.
That will be a very hard number to find. Mobile is mostly a controlled access kind of thing, but desktops and servers have long been a statistically awkward free for all. I write software as a professional developer targeting mostly linux servers, but I have a bit of deployed code out there in the wild running on windows servers. However, you're not very likely to find my work on any statistical analysis database, and you're not even going to find me listed anywhere as a normal business.
Plus there is also the awkwardly nebulous nature of software developers. I know a group of five developers, and between them they have I think eight 'companies' offering different products and services. Do you count them as five developers, or eight? Or eight companies with 2-4 developers each?
We are looking for some rough figures for potential customers. I suppose something like number of full-time workers or something.