This function will return the row index starting from 1 (first row = 1, second row =2, etc).
<?php if( have_rows('slides') ): ?>
<?php while( have_rows('slides') ): the_row(); ?>
<div class="accordion" id="accordion-<?php echo get_row_index(); ?>">
<h3><?php the_sub_field('title'); ?></h3>
<?php the_sub_field('text'); ?>
</div>
<?php endwhile; ?>
<?php endif; ?>