Google Fuchsia's kernel is 199,570 lines of C, C++, and assembly, not including a suitable C/C++ toolchain (it uses GNU extensions, so we'll assume GCC is necessary - about a million lines of code). It supports two architectures, x86 and arm64. It takes about 22 minutes to build on server-grade hardware. It also doesn't work and is far from complete.
The entire self-hosting plan 9 system, including its compiler and entire userspace, desktop windowing system, text editors, POSIX compatibility layer, games, media players, and so on - is 268,001 lines of code. It supports seven architectures and takes less than 10 minutes to build the entire operating system on a raspberry pi.