I think I understand, you want to assign a certain number of "Credit Hours" or credits to a course, and then have the certificate list how many credits that course was worth, right?
Sounds like the first step is to figure out how to determine the number of credits each course it worth and how to add that to the database.
I would start with modifying the course settings page (../course/edit.php), add a new filed in the database and a new "Credit Hours" input filed on the course settings page. Once you have the number of credits that a course is worth in the database it's fairly simple to get it to print on the certificate.
I would like to say I can tell you exactly how do to that but I'm still learning PHP myself so hopefully one of the more experienced programmers on this site could help with the details of how to do what I described.
If you get the credits saved in the database I can help you with modifying the certificate to print the credits on it.
Hope this helps a little.