Schools will want to see that you are familiar with the profession and have put some time into getting to know it.
But a direct answer to your question is tough--it is going to depend on the type of shadowing.
For example, 100 hours of standing in the corner, 8 feet away from the patient, with a dentist who never speaks to you, will pale in comparison to 20 hours of being actively involved with a dentist, having the dentist explain all the instruments to you, and having the dentist ask you what treatment plans you can come up with.
100 sounds a whole lot better than 20 hours, but that isn't always the case. It's the quality of hours you should be going for, and those quality experiences should translate over in your interview, regardless of the actual number of hours you shadowed.