Only ~20k lines. From the book's into:

> The current Linux kernel source code amount is in the number of millions of lines, the 2.6.0 version of the kernel code line is about 5.92 million lines, and the 4.18.X version of the kernel code is extremely large, and it has exceeded 25 million lines! So it is almost impossible to fully annotate and elaborate on these kernels. The 0.12 version of the kernel does not exceed 20,000 lines of code, so it can be explained and commented clearly in a book

1 million lines of code in 1000 pages would come out to 500 lines of code per side of page. You'd need a big magnifying glass to get through that.

The book covers an old version of linux which had a few thousand lines of code.