how to rename multiple picture files at once

I don't believe you can since you cannot have duplicates - for instance picture #1 is fine, the next one needs to be #2 though or a different title, etc. As far as I know, Mac OS or any app I've used will not do that automatically - I don't know if there are any third party apps that will.

You cannot rename multiple files in Preview. You could do Save As ... to rename files, but that only works with one file (image) at a time and isn't actually renaming, it's saving a duplicate with a different name.

For batch renaming, I recommend A Better Finder Rename. I have used this over a number of years and different releases. It was excellent to begin with, and gets even better with each release. There is also a companion app called A Better Finder Attributes that I have found helpful with image files also ... creation & modification dates, EXIF info, etc.