Использование find:
find . -maxdepth 1 -type f -print0 | while read -d $'\0' f; do mv "$f" "$f"_$(grep -c . "$f"); done
Пример
% wc -l *
3 doit
5 foo
% find . -maxdepth 1 -type f -print0 | while read -d $'\0' f; do mv "$f" "$f"_$(grep -c . "$f"); done
% wc -l *
3 doit_3
5 foo_5