Intel uses a specialized version of Minix 3 for its Intel Management Engine in every CPU it released since 2008, so technically, it is the most installed OS.
Even if Minix had been installed on every single management card on every computer Intel made from then to now, that number is utterly dwarfed by the number of systems with an installed Linux kernel. That's because in addition to its use as a server and desktop OS, the Linux kernel is the core of the Android distributions that almost every non apple smart phone uses today. Also set top boxes, tablets, watches, and even many smart TVs and Blu Ray players have a copy of Android.
The Linux kernel is a kernel, not an OS. Even if my set-top-box is running some flavor of Android, it looks nothing like my phone or anything else, thus I would classify it as a different OS -- even if the underlying code is based on Android.
Even if Minix had been installed on every single management card on every computer Intel made from then to now, that number is utterly dwarfed by the number of systems with an installed Linux kernel. That's because in addition to its use as a server and desktop OS, the Linux kernel is the core of the Android distributions that almost every non apple smart phone uses today. Also set top boxes, tablets, watches, and even many smart TVs and Blu Ray players have a copy of Android.