You're probably right; most instances of operating systems running out there must lack support for dark mode. Just think about the operating systems in our fridges, thermostats, servers, and similar; dark mode just doesn't make sense in many places.
I think it might look slightly different when it comes to dark mode support on devices that run web browsers. Around 92 % of the global web surfers use browsers with support for dark mode. When it comes to operating systems Android, Windows, iOS, and macOS – all of which have added support for dark mode in recent versions – make up around 96 %.
I think the actual percentage will be lower. Not all users can (or want to) run on the latest OS version. But I think it's safe to bet a large chunk of web dwellers have support for dark mode on their devices. Maybe even a majority. When it comes to how many choose to enable dark mode, well, that's probably not that many. 😅