"Mogrify" should be called from the directory with the original thumbnails, while the -path parameter is for pointing target directory.
cd public_html/images/thumbs
magick mogrify -resize 16x12 -quality 100 -path ../new-thumbs *.jpg
http://www.imagemagick.org/Usage/basics/#mogrify
the last arguments are the list of files, so you can filter by name pp*.jpg for example.