Have a look at the contents of your backup folder. The number of items shown on the bottom status line of the organizer shows the number of media files (photos or videos). Version sets or stacks are considered a single image. If you shoot raw, each file gets its own xmp sidecar file. Other features like face recognition also create additional text files.
So, it's best to check the number of files from your explorer or finder rather than trusting the number of images shown by the status bar in the catalog. The total size (kilobytes) of the media in your explorer should roughly match the required available size shown in the beginning of the backup process.