Counting Comments as Singular and Plural in comments.php Template

This should do the trick…

<?php
    $number_of_parents = comment_counter($post->ID);
    $number_of_children = $post->comment_count - $number_of_parents;
?>
<?php if ( have_comments() ): ?>
    <h3 class="comments-title">Discussion: 
        <?php if ( $number_of_children > 0 ): ?>
            <?php echo implode( ' ', [
                printf( _n( '%d question', '%d questions', $number_of_parents ), $number_of_parents ),
                __( 'and' ),
                printf( _n( '%d answer', '%d answers', $number_of_children ), $number_of_parents )
            ] ); ?>
        <?php else:  ?>
            Questions: <?php echo $number_of_parents; ?>
        <?php endif ?>
    </h3>
<?php endif ?>