If you're a recent grad or unemployed, sure, apply to 20 a day. But if you're working full time and trying to make a switch, a more reasonable target is 2-3 job applications per day, max. Any more than that means you're probably not giving each application the amount of attention it deserves.
You may be a great candidate, but if you're not taking the time to highlight specific relevant skills, it's easy to see why your resume might get overlooked. Job hunting is more than just replying to as many posts as possible with your generic resume and cover letter attached. Instead, you should be researching every company you apply to and tailoring the application to the specific job -- and maybe even finding out who you know that knows someone who works there. If you're applying for similar jobs, your standard resume will probably work for all of them, but you should at least write a new cover letter every time.