The typical recessed wood medicine cabinet with mirror will come with two shelves. In most cases, this will be enough. However, if you are only planning to put small items in the cabinet, you can go for more shelving.
If you're not sure how many shelves you'd like, then you might want to opt for adjustable shelving. This feature will make the cabinet a bit costlier, but it will save you the trouble of returning the item in case the number of shelves doesn't work for you.