There isn't really a set amount, however, some universities may have guidance on how much they would like you to have, it really depends where you are thinking of applying too.
Personally I am a current medical applicant and over the last few years have tried to get quite a lot. Some placements may only be a half day in each department but it adds up and gives you a good taste of medicine.
When writing about it in your statement and speaking about it, reflect on your experiences so keep a record of what you did, saw and found interesting, etc.
The medic portal have compiled this, which may help, definitely also look up on the websites of the universities you may be considering as they may have guidance:
https://www.themedicportal.com/application-guide/work-experience/medical-school-requirements-medicine-work-experience/I think there are a few universities that say 2 weeks equivalent of medical related work experience, while somewhere like Edinburgh says they value quality over quantity so it depends.
I think I have like 2 weeks equivalent hospital experience- this is from several placements in different departments and hospitals over a few years though- ranging from half a day to 4 days in each department.
I also volunteered on a dementia project though my school (while in S5) and am currently volunteering with children suffering from Autism.
I would say try to get what you can (some placements take a while to secure and may not come through- happened to me), but look if your universities have specific requirements.