Visiting Shanghai and Hong Kong, do I need multiple or Single visa for my trip?

I am a Korean Citizen in NY, and I am planning to visit Shanghai and Hong Kong this break. So my schedule is like, NY - Seoul ( 3 days) - Shanghai ( 2 days) -Hong Kong (3 days) - NY
Q2. If I change the plan to visit Hong Kong only, do I need to apply still?
Q3. Do I need anything other than application, photo and passports, like my flight itinerary?