Invitation and numbers of entries

Hi, I've an invitation from a university to come to China for four-five months. It says that I'll have an single entry. Since I'm interested in visiting Hong Kong and neighbouring countries while in the area, I'm wondering if I need to ask the university to change the number of entries or have them leave that part out all together? Or does it matter to the application process? If I have multiple entries, can I exit and enter as i choose or is it regulated?