Here what it says in the assignment: "Create a class diagram. It is not necessary to provide each method name and attribute in the class. " It is talking about one class diagram. In my desig, I have many class diagrams seperated by use cases and front/middle tiers. What was your approach, one diagram would be very crowded and hard to follow.
from previous posts I know that people have got good grades by making one and by making more than one. I don't think the number itself is important, it is rather that it should not be too detailed, like you were doing design...