I’ve managed to get @hube2 solution to work, but with help from (for me) a bit clearer code:
You’ll need to start a counter just above your while loop:
$counter = 0;
Then just before your closing endwhile; you’ll need to advance the counter:
$counter ++;
Then simply print the number of that loop like this:
<?php print "$counter"; ?>
or
<div class="name-<?php print "$counter"; ?>"
Source: https://support.advancedcustomfields.com/forums/topic/repeater-field-add-break-after-x-rows/