global $post;
$id = $post->ID;
$post = get_post($post);
$images =& get_children( 'post_type=attachment&post_mime_type=image&output=ARRAY_N&orderby=menu_order&order=DESC&post_parent='.$post->post_parent);

$galry = get_post_gallery( 1234, false );
$images = [];
if ( $galry && isset( $galry['ids'] ) ) {
    $ids = wp_parse_id_list( $galry['ids'] );
    $images = get_posts( [
        'post__in'  => $ids,
        'post_type' => 'attachment',
    ] );
    echo 'Number of images in gallery #1:<br>';
    echo count( $images ) . '<br>';
}

$galrys = get_post_galleries( 1234, false );
foreach ( $galrys as $i => $galry ) {
    $ids = wp_parse_id_list( $galry['ids'] );
    $images = get_posts( [
        'post__in'  => $ids,
        'post_type' => 'attachment',
    ] );
    echo 'Number of images in gallery #' . ( $i + 1 ) . ':<br>';
    echo count( $images ) . '<br>';
}