There's really no set number that you need to strive for, I think the thing that they value more is the type of experiences that you have. Are you just standing around and not really getting anything out of it or were those hours well spent being proactive, getting a better assessment and appreciation for what being a dentist entails.
They will always look to your grades before they'll even look down at how much shadowing experience you may have so I would keep your GPA as high as possible to better your chances at getting interviews.
With that being said, I do think that if you can squeeze in some time to shadow/volunteer you can only help yourself in the long run.
1) you can decide whether you really want to enter this profession (school is not cheap and it's hard as hell, why waste your time if you're not 100% certain?)
2) if you really bond with a dentist, he/she can provide mentorship and a REALLY EXCELLENT letter of rec (you need to start finding people that are going to set you apart from your avg. "Joe is a great guy and he came to the office 3 times last summer" kinda letters.)
3) a slight leg up of the procedures before you even start (I got personal training on how to do wax-ups, taking impressions, etc.)
4) you can also get a feel for the different specialties within dentistry. Sometimes you end up changing your mind and become more attracted to other fields as you go through your training but at least you can get a preview of what they are. I knew I wanted Peds before I started school and that kinda gave me a goal and direction as to having to do well in the classroom and on the boards.
