You cannot assume that /bin/sh always refers to Bash. There's a lot more to Unix than just Linux and Mac OS X. On FreeBSD /bin/sh is a 1989 rewrite of the SVR4 Bourne shell. As of 10.0-RELEASE it comes to 21167 lines, including its Makefile, ancillary shell scripts, and documentation.

That's not really fair since the shell is an independent part of the system and could be swapped out for another implementation. Plus, it's not there just for the init system; it'd be there regardless of the init system. It's an already existing independent component, which is used for leverage.

Otherwise should we also count the C compiler, libc, all the CLI utilities and the kernel?

But here you go. Still smaller than systemd. Sysvinit needs a shell too...

    $ wc -l /usr/src/bin/ksh/*.[ch]
     127 /usr/src/bin/ksh/alloc.c
    1412 /usr/src/bin/ksh/c_ksh.c
     935 /usr/src/bin/ksh/c_sh.c
     560 /usr/src/bin/ksh/c_test.c
      53 /usr/src/bin/ksh/c_test.h
     201 /usr/src/bin/ksh/c_ulimit.c
      60 /usr/src/bin/ksh/config.h
     829 /usr/src/bin/ksh/edit.c
      86 /usr/src/bin/ksh/edit.h
    2163 /usr/src/bin/ksh/emacs.c
    1333 /usr/src/bin/ksh/eval.c
    1433 /usr/src/bin/ksh/exec.c
     107 /usr/src/bin/ksh/expand.h
     593 /usr/src/bin/ksh/expr.c
     984 /usr/src/bin/ksh/history.c
     438 /usr/src/bin/ksh/io.c
    1653 /usr/src/bin/ksh/jobs.c
      13 /usr/src/bin/ksh/ksh_limval.h
    1643 /usr/src/bin/ksh/lex.c
     132 /usr/src/bin/ksh/lex.h
     196 /usr/src/bin/ksh/mail.c
     786 /usr/src/bin/ksh/main.c
    1149 /usr/src/bin/ksh/misc.c
      91 /usr/src/bin/ksh/mknod.c
     285 /usr/src/bin/ksh/path.c
     267 /usr/src/bin/ksh/proto.h
     420 /usr/src/bin/ksh/sh.h
    1161 /usr/src/bin/ksh/shf.c
      82 /usr/src/bin/ksh/shf.h
     897 /usr/src/bin/ksh/syn.c
     231 /usr/src/bin/ksh/table.c
     183 /usr/src/bin/ksh/table.h
     420 /usr/src/bin/ksh/trap.c
     708 /usr/src/bin/ksh/tree.c
     141 /usr/src/bin/ksh/tree.h
      57 /usr/src/bin/ksh/tty.c
      37 /usr/src/bin/ksh/tty.h
    1210 /usr/src/bin/ksh/var.c
      10 /usr/src/bin/ksh/version.c
    2128 /usr/src/bin/ksh/vi.c
   25214 total