"Lots" is a relative term, but the overwhelming majority of kernel developers are employed and usually do kernel work as part of their job (usually at least ~80% but it could be argued as high as 97% depending on how you interpret the breakdown done by LWN of each release[1]).
And I would guess that most of the kernel devs who are "working for free" are doing the stuff they personally enjoy and find satisfaction in working on, because it's a hobby -- so many of them are probably not interested in fixing random bugs for cash either.
[1]: https://lwn.net/Articles/1038358/