For PDFs this function indicates the number of pages on the PDF, NOT images that might be embedded within the PDF.
<?php
$document = new Imagick('2_pager.pdf');
var_dump($document->getNumberImages()); //returns 2
$document = new Imagick('1_pager.pdf');
var_dump($document->getNumberImages()); //returns 1
?>
From my research it is not possible to obtain the images embedded within a PDF by using the PHP Imagick library.