Count total number of images in post and echo results as number

Use regex to find all the urls and filter by type.

$post    = get_post( 504 );
$content = $post->post_content;

// match all urls
preg_match_all( '/(http:|https:)?\/\/([\w_-]+(?:(?:\.[\w_-]+)+))([\w.,@?^=%&:\/~+#-]*[\w@?^=%&\/~+#-])?/', $content, $matches );

$count = 0;
if ( ! empty( $matches ) && ! empty( $matches[ 0 ] ) ) {
    foreach ( $matches[ 0 ] as $url ) {
        $split    = explode( '#', $url );
        $split    = explode( '?', $split[ 0 ] );
        $split    = explode( '&', $split[ 0 ] );
        $filename = basename( $split[ 0 ] );
        $file_type = wp_check_filetype( $filename, wp_get_mime_types() );
        if ( ! empty ( $file_type[ 'ext' ] ) ) {

            // (optional) limit inclusion based on file type
            if( ! in_array( $file_type[ 'ext' ], array('jpg', 'png')) ) continue;

            $files[ $url ] = $file_type;
            $urls[]=$url;
            $count ++;
        }
    }
}

// print out urls and total count
print_r( array ( 
        'total'  => $count, 
        'unique' => array_keys( $files ),
        'urls'   => $urls 
) );

---

OOP

If you want this as a reusable function…

function get_file_urls( $content="", $file_types = array ( 'jpg', 'png' ) ) {

    // match all urls
    preg_match_all( '/(http:|https:)?\/\/([\w_-]+(?:(?:\.[\w_-]+)+))([\w.,@?^=%&:\/~+#-]*[\w@?^=%&\/~+#-])?/', $content, $matches );

    $urls  = array ();
    $files = array ();

    if ( ! empty( $matches ) && ! empty( $matches[ 0 ] ) ) {
        foreach ( $matches[ 0 ] as $url ) {
            $split     = explode( '#', $url );
            $split     = explode( '?', $split[ 0 ] );
            $split     = explode( '&', $split[ 0 ] );
            $filename  = basename( $split[ 0 ] );
            $file_type = wp_check_filetype( $filename, wp_get_mime_types() );
            if ( ! empty ( $file_type[ 'ext' ] ) ) {

                // (optional) limit inclusion based on file type
                if ( ! in_array( $file_type[ 'ext' ], $file_types ) ) {
                    continue;
                }

                $files[ $url ] = $file_type;
                $urls[]        = $url;
            }
        }
    }

    // print out urls and total count
    return array (
            'total'        => count( $urls ),
            'urls'         => $urls,
            'total_unique' => count( $files ),
            'unique'       => array_keys( $files ),
    );
}

$post      = get_post( 504 );
$content   = $post->post_content;
$file_urls = get_file_urls( $content, array ( 'jpg' ) );
$count     = $file_urls[ 'total' ];

echo "<div class="count">${count}</div>";