The GNU utils version ([github](https://github.com/coreutils/coreutils/tree/master/src/wc), [savannah](http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=blob;f=src/wc.c;hb=HEAD)) is a bit over 1K lines of C. It does many things and checks many possible failure modes.
The busybox version ([git.busybox.net](https://git.busybox.net/busybox/tree/coreutils/wc.c)) of wc is much shorter, at 257 lines, while striving to be [POSIX-compliant](https://pubs.opengroup.org/onlinepubs/9699919799/), meaning it has flags.
The plan9port version of wc ([github](https://github.com/9fans/plan9port/blob/master/src/cmd/wc.c)) implements some sort of table method, in 352 lines. So does the [plan9](https://9p.io/sources/plan9/sys/src/cmd/wc.c) version, which is worse documented, but shorter.
[Here](https://github.com/dspinellis/unix-history-repo/blob/Research-V7-Snapshot-Development/usr/src/cmd/wc.c) is a version of wc from UNIX V7, at 86 lines, and allowing for both word and line counts. I couldn't find a version in UNIX V6. Of all the versions, I think I understand this one best.