Some schools that set minimum required hours for activities such as shadowing, but in general, most schools do not have set hours that you need to achieve. If you are applying somewhere that does have set requirements, aim to come in with more hours than the minimum.
With each experience, it isn't as much about the hours logged as it is about what you learned or gained from that experience. With that said, aim to devote 100+ hours each to shadowing, hands-on experience in a clinical setting, and community service.