Something like this? 🙂
<?php if( have_rows('questions') ): ?>
<?php $counter = 1; //this sets up the counter starting at 0 ?>
<ul class="questions-answers">
<?php while( have_rows('questions') ): the_row(); ?>
<li><h3><a href="#"><?php the_sub_field('question_text'); ?></a></h3>
<p class="class_<?php echo $counter; // Prints the number of counted row ?>"><?php the_sub_field('answer_text'); ?></p>
<p><?php the_sub_field('answer_text'); ?></p></li>
<?php $counter++; // add one per row ?>
<?php endwhile; ?>
</ul>
<?php endif; ?>
