I am going to make up three similar sentences below.
Do you have a class today?
Do you have class today?
Do you have classes today?
I think (1) doesn't make sense because I haven't heard of anyone saying that. (2) is correct because you are talking about the class you are taking right now. (3) is also correct because you are thinking about all the classes you are currently taking. Is my interpretation correct? What is your opinion?