He is on the LKML[0], and does respond to many people, not just the handful of maintainers. I'm sure he doesn't read the entirety of the LKML (hundreds of messages per day), and for sure he has a private channel (mailing list or otherwise) with subsystem maintainers, but he DOES correspond with a lot more than that handful, and when I was reading the LKML, he would even occasionally respond to first-time posters.

[0] https://en.wikipedia.org/wiki/Linux_kernel_mailing_list