I am planning to visit Zhongshan then go to Sanya for some 7 days. Do I need 2 entry visa if I want to go back Guangzhou after that?

Hi everybody!
Is Hainan Island regarded as "outside mainland China"? I am planning to have a business meeting in Zhongshan then go to Sanya for some 7 days. Afterwards I will go back to Guangzhou to fly back to my country. Do I need to have 2 entry visa or a single entry one is OK in this case?