Some schools dont require it, others do. I think if you're going into ANY health-related career, it's good to have some volunteer experience throughout your time before applying. It's great to offer service and help others out, and having those interactions early on will be beneficial for nay health career.
If you need a number in mind for specific purposes, I think anything over 100 hours shows commitment and desire to help others in the community, which I'm sure ADCOMs will love to see; plus, it will give you a good foundation for dentistry, as well as something to talk about.
Good luck!