What I would say to this is, the more the better! Medical schools want to see that you truly understand how the medical field works and want you to know what kind of career you’re getting yourself into (hours, expectations, responsibilities). In terms of a minimum number of hours that you should volunteer, check the websites of the medical schools you plan to apply to! They often suggest how many clinical hours they would like you to have before applying to their program.
 

It is not necessary that you volunteer in a hospital to gain clinical hours, unless the medical school(s) you are planning on applying to specifically say so (however, most medical schools are flexible in where you gain your clinical hours). So long as you are interacting with “patients” (a.k.a. an individual receiving medical care), you should be good to go.

— Margarete