Theoretically, you can take the MCAT as many times as you want. After 3 tries, you have to send a rejection letter to the AAMC to show that you're still trying to get into med school to get another shot. Most schools will look at and consider all of your scores, though a few of them will only look at the highest or most recent scores. A couple schools (Utah and GW are the only ones that spring to mind) will do like the SAT and look at your highest scores from each section if you've taken the test multiple times. Taking the MCAT more than once if you've already gotten a good score or a score that matches up well with your practice scores is probably unwise since you stand a pretty good chance of doing worse, and that looks bad.