Schools vary in what they will require. I've seen anywhere from 0 to 30 to 100. To be competitive, I would shoot for over 100! I personally did about 140 hours. Getting a position is pretty simple. About a month before you want to start interning/shadowing, call dentists in the town you will be at and explain who you are, if the doctor is available for an interview, if you can send the doctor your resume, etc. If your town is very impacted (like my college town - I called like all dentists in my town and only one gave me a spot since my college town has all these students competing for internships), then call dentists in towns bordering your city. It's easy if you have a car. When you call, be nice, pleasant, and genuine.
And even though you have all those hours for volunteering, extracurriculars, research - you STILL need to shadow dentists. It's a completely different requirement and section altogether on your dental school application when you eventually apply. That being said, continue with whatever you are doing and start shadowing!