Rename to 'file' of 'total files'

Install this new script command:
RenameEx: Command: RenameEx (extended Rename command)

Regarding your topics:

  1. Create a button or call:
    RenameEx NUMBER PAD TO="{stem} of {totalsel}{ext}"

  2. Create a button or call:
    RenameEx NUMBER PAD TO="{dlgstring|New filename:|{file}} - {num} of {totalsel}"