This (fairly long) one-liner counts the number of implementations of each hook in your Drupal installation:
ack "Implements? hook_" | perl -e 'while (<>) { m[(hook_\w+)] and $hooks{$1}++; } foreach (keys %hooks) { print "$_ - $hooks{$_}\n"; }'To count only install file hooks, which was what I was doing, give ack the option "-G '.install'" thus:
ack "Implements? hook_" -G '.install' | perl -e 'while (<>) { m[(hook_\w+)] and $hooks{$1}++; } foreach (keys %hooks) { print "$_ - $hooks{$_}\n"; }'