Single entry Visa vs. Double entry

I am going to China with a group from my college in the USA, I know I need a single entry to enter. While we are there we have plans to do a day excursion to the Island of Macau. Some sources tell me I need a multi entry to reenter China after my visit to Macau and other say I do to a single entry is all I need. Please help and clarify which type I need.