They get 30-40 on average. It takes me the better half of the day to review them all. People have work to do; you don’t. Customise your resumes for the application and sometimes that means a letter in certain situations goes a long way.
If you’re sending out the same resume you spent a few hours doing, week after week, you’ve done five hours of work in several weeks. You can’t expect people working 40 hr work weeks, where reviewing resumes is an addition task, to then start writing personalised responses to every person.