How to modify the “after” of last element in wp_nav_menu

When using wp_nav_menu how do I make it so ‘after’ => does not show on the last element of the list?

You can hide it with CSS. For example, if your after looks like:

<?php wp_nav_menu(array('after' => '<span>|</span>')); ?>

Then your css:

.menu-item-num span { display: none; }