When developing a course, how to determine number of credits?
Your institution almost certainly has guidelines for what constitutes a "credit" - you will have to rely on those guidelines.

For example: http://archive.catalog.arizona.edu/2015-16/policies/credit_definitions.htm

At US universities that I am familiar with, the common definition is that 1 credit = 1 weekly 'hour' of class time, and ~2 weekly hours of effort outside of class, giving a total of 36-45 hours a week for a full-time load of 12-15 credits. Of course for some types of classes, these numbers get shifted around, and they are totally different in courses not over a full semester (though the effort is the same, the time is much less).