Think about your audience. They are asking this question because they want to know how you'll contribute to the teaching of the department. Only listing the title of the course would provide a little information (what you'll teach), but probably not all of what they're listening for (how you'll teach or how your teaching interests will fit in their department).
So, for each course you would answer for, be ready to give at least two sentences that focus on any of the following:
Key learning objectives - what would you focus on?
Motivation - why does designing this course interest you?
Fit - how do you see this course fitting in? Is it for majors? Is it a core requirement? Is it a special topics course?
Key curricular approach - how would you teach it?
Which elements you choose depends on you, your field, and the program you're applying to. For me, if it was an established course that's part of their own major curriculum, I'd talk about motivation or curricular approaches, since it's not like I anticipate changing the learning objectives radically at the start. If I'm proposing something not in their major plan, I'd want to address fit, because it may not be obvious how I see the course contributing to their department. If it's a fairly broad course like a survey, I'd speak more to learning objectives or (again) curricular approaches, since people can approach surveys in a variety of ways. In other words, I explain what I think would be either most interesting or least obvious to my audience.
Also, consider the balance of speaking to what you'd like to do and what the department needs in the next year or two. They do want to hear the former, but it may also help if they know you'd be interested in teaching a workhorse course that's always in demand. If you can communicate that you'd teach that workhorse course in an interesting way, even better.
The number of courses to describe is dependent on field. In English, I had a freshman composition course, a sophomore-level survey, and an upper-division course ready to go, and I'd talk about 1-2 of them in any given first interview, and then add examples of other courses to teach at the end of a question. I'd take about 2 minutes total with that question, and select the courses based on research.