Shadow MDs or DOs only
Don't ask for a specific number of hours to shadow for, that's weird. Ask to shadow for like one day per week for a month or something like that, then count up the number of hours that you ended up shadowing. In terms of med school admissions, it's not very important how many hours you shadow, its much more important how you present what you gleaned from your shadowing. You should be able to make an argument that you have seen the day to day life of a doctor and that's something you'd be interested in doing for the rest of your professional life. Some people can demonstrate this with fewer shadowing hours, some people may need more - also may depend on the quality of the shadowing hours. Many people will shadow for between 40 and 80 hours to be able to cogently express this. Some people only shadow for 20, some people shadow for over 100. A person who shadows for 150 hours isn't going to have any sort of leg up on someone who shadowed 50 hours if they can both convey the same idea to admissions committees (via interviews, personal statement, etc).
Letters of recommendation from doctors you've shadowed are worthless except in 2 situations. The first and most common is if you are applying to a DO school, in which case you'll need a rec from a DO. The second which is much less common is if you are applying to one of the 4 or so MD schools that requires a physician letter of rec (I don't know which these are, do a search and you'll find them). In any other scenario, a physician LOR will be worthless because there is no character quality they can really say about you other than "was attentive, present, and asked questions at appropriate times" - this, while 'positive' is a weak letter and will almost certainly be weaker than any other letter you get (from a professor, from a research mentor, from a volunteer coordinator, etc). Thus, presenting a shadowing letter may imply that you were unable to acquire a stronger letter, which does not shed a positive light on your application.
