It's monolithic but modular 11.5M LOC for every platform it runs on, not to mention drivers for the devices it supports, and all the modules for other functionality, pluggable file-systems and networking stacks. Now throw in code for backward compatibility going further back at least to 2.4 and you have 11.5M lines.

In reality, the core Linux kernel is just the stuff that compiles to vmlinux. You could read it all in a few weeks and understand it.