Passport'll be stamped

US immigration officer will stamp both on AP & passport.They don't require you have a full blank passport page. As long as your passport has some blank space for stamping, you are fine.
Just a reminder: Chinese custom officer will stamp twice(in/exit) on your passport as well. So if you really run out of space(no place to squeeze the three stamps), better to get a new passport. It should not make things more complicate for your GC application process. Just take both when you get GC stamp when your case is approved. Good luck!