50 hours of shadowing and 150 hours of non clinical volunteering are considered baseline. The shadowing should include some primary care, and the volunteering should be with underserved or disadvantaged groups.
Once you meet the shadowing baseline, whether you pursue more is entirely your choice—tons isn’t necessary. For the volunteering, the more the merrier. With your heavy research experience, you’ll want to get a healthy dose of volunteering in to counteract any perception that you’d prefer to be a PhD over MD, unless you go MD/PhD route.