There can be a huge difference and there can be a small difference. For example, you can be "volunteering" at a dental clinic at a hospital, in which case you're probably only handling paper work and working at the reception desk. But if you're "shadowing," you're actually inside the clinic, observing all the procedures and interactions between the patient and doctor. Sometimes though, if you're assisting and actually working (such as sterilizing instruments, running errands, cleaning the clinic, etc.) and you're not getting paid, then they may call you a volunteer, even though you're getting the shadowing experience as well. So basically, whatever the title is, make sure you get some exposure to dentistry being performed besides paperwork.
In terms of how much you need of each, it depends... the more the better. I don't think there's a figure that you can call minimum or maximum.
I guess there are many ways to interpret the same thing. I think shadowing is watching dental procedures from the side and volunteering is actually helping out, maybe setting up the instruments or autoclaving them afterwards. Or even pouring the mold for a tooth impression.
Regardless of how you define shadowing vs. volunteering make sure you explain yourself when you write it on your secondary or AADSAS application. Make sure to explain what you did at the dental office or clinic and then call it accordingly (shadowing or volunteering).