It's not just how many you get wrong, but how hard the questions are and where they are in the test. The GMAT is adaptive, so the level of difficulty of the questions you see is based on whether you got the previous one right or wrong. Each section of the test starts with a medium-difficulty question, and the test hones in on your ability in the first 10 or so questions. So it's really important to get those right -- spend more time on those initial questions.
Without knowing where the questions you got wrong are within your math section, it's hard to say how you were penalized, but PR's tests are adaptive in the same way as the real GMAT, so it should give you a realistic idea. But...I wouldn't get try to gauge your future success on one practice score. Your score will fluctuate as you study (hopefully not too much!) depending on how prepared you are, how tired you are, etc.
Hope this helps...