Hi @chrisfry
Sounds like you need to use a counter variable to dynamicaly echo the row number. You can do this like so:
<?php if(get_field('menu_item')): $i = 0; ?>
<?php while(has_sub_field('menu_item')): $i++; ?>
<div id="test-<?php echo $i; ?>">
</div>
<?php endwhile; ?>
<?php endif; ?>
Note that the variable $i will increment each row, and you can render it with the echo function.
Does that help?
Cheers
E