Due to COVID these days, many physicians are not accepted students as willingly due to safety precautions (but not all). For your med school application, some schools require a letter of rec from a doctor you have shadowed for a minimum amount of hours (can be anywhere from 20 hours up to 60 hours). In this case, its helpful to shadow 1 doc for a consistent period of time to develop a relationship and get that letter of rec. My advice is it would be wise to target 1 and put your effort into that. However, shadowing a couple more doc in different fields (psych, family med, pediatrics, or another specialty) can be helpful if you experience something you want to put in your personal statement.
Minimum one if you are struggling to get any shadowing experience, but recommended you try to shadow 3 different doctors to get some experience.
Hope this helps