I would like to be able to display the amount of karma a single post has received from members. What would I need to add to Display.template.php to do this?
I'm not sure if I understand your question completely. There is a feature which allows members to have a karma value. That can be enabled in admin panel > Core Features.
I found a mod which appears to allow you to have karma for posts (or maybe topics?). Note that I haven't used it. But that's what it sounds like it does: https://custom.simplemachines.org/mods/index.php?mod=3474
a post does not receive karma, a user receives karma, so you cant really get a number the way you want.