So, for example, you want to sell packages of 5 fish, and have the inventory system treat that as 5 fish and not 1 package of 5 fish?
You can edit the quantity.php template file under single-product/add-to-cart/ (under your theme) and use the step attribute along with a meta field, so it would look something like this:
<div class="quantity"><input name="<?php echo $input_name; ?>" data-min="<?php echo $min_value; ?>" data-max="<?php echo $max_value; ?>" value="<?php echo $input_value; ?>" size="4" title="Qty" step="<?php echo get_post_meta(get_the_id(),'package_size',true);?>"class="input-text qty text" maxlength="12" /></div>
if you set meta field "package_size" to a value of 5, then the + button would count by fives.