Do I need a Single Entry or Multiple Entry Visa to China?

I am an American. I will be traveling first to Hong Kong, then to mainland China for 2 weeks, then to Thailand and then back to Hong Kong in order to return to the US. I believe that I only need a Single Entry to China as I will not be returning to mainland China, but only to Hong Kong for an overnight stay. Is a Single entry sufficient OR should I get a Multiple Entry?