I've looked at the 9th, 10th, and 11th editions of the
OG. The 10th edition contains the 9th edition plus a lot more problems, so forget the 9th edition. The 10th edition contains many more questions than the 11th edition, but from what I understand, extra questions from the 10th edition were taken out and used in software, which one I'm not sure, because I've heard there are two sets of software, one from Pearson and one owned by GMAT. On another thread on this forum, a person says that the
OG 12th edition also introduces 300 more questions too. I don't know personally though.