There are a lot of people who are paid to work on the kernel full time from Red Hat, Google, IBM and many others. If I had to guess I'd say there are probably more than the other two, it would be interesting to find out. But if you include people where it's not 100% of their job, but still an official part of their job, I'd say it's almost certainly more for Linux (not even counting unpaid contributions).
Linux is the most popular platform for servers and HPC because most of the time it's the better kernel. It's so dominant that Apple has basically left the server area and the handful of showcase supercomputers built on Apple gear have long since faded from view. Linux went from having one supercomputer in the top 500, a fraction of a percent, in 1998 to 98.8% of the top 500 currently.[0] The other six seem to be IBM machines running something else. OS X's first release was 2001 and it and Microsoft offerings are simply not present in that top section of the HPC space.
Linux is also the kernel on the most popular smartphone platform so it's not all computational either, when the hardware is tightly controlled it works fine. The problem isn't the number of developers on kernel or how hard it is to debug, it's that laptops and desktops aren't offered from a single company that can tie everything together.