As of this morning in the Linux kernel Git source tree, the kernel weighs in at 27.8 million lines! The stats as we start 2020 put the Linux kernel source tree at 27,852,148 lines (including docs, Kconfig files, user-space utilities in-tree, etc), 887,925 commits, and around 21,074 different authors. The 27.8 million lines are divided among 66,492 files.
During 2019, the Linux kernel saw 74,754 commits, which is actually the lowest point since 2013. The 74k commits is compares to 80k commits seen in both 2017 and 2018, 77k commits in 2016, and 75k commits in both 2014 and 2015.