I would say that everyone should do at least three practice exams, so they can get a feel for the test and get their timing down. Every practice test should be taken under real exam conditions, with the exact time constraints, even down to the breaks and lunch time. The tests should also be high quality practice tests. Preferably AAMC IV-VI (or the equivalent if they wind up changing the practice tests to match the new test format), followed by any of the big prep companies (Kaplan, TPR, EK, Berkeley Review, Columbia Review), and I would avoid many of the bookstore tests (REA, Barrons etc.). Every exam should be thoroughly reviewed after it is taken. Go over every question, especially the ones you got wrong. Figure out what makes a question wrong or right, and if you could have answered the question more quickly or more efficiently.
If you're scoring where you want after 3 exams, it's probably not necessary to take more, but practice never hurts.
My personal opinion is that most people should take between 6 and 8 practice exams. Again all of these should be under exam conditions, and all of the tests should be carefully reviewed afterwards. You should always be looking to improve your test-taking skills.
I think that 20 practice exams would be overkill for most people, but like I said before practice never hurts. Just make sure you don't spend your time taking more tests, rather than effectively reviewing the tests you've already taken or learning the necessary science. Like efex101, I planned on taking about 20 practice tests. I had bought 8 Berkeley Review tests, 7 Columbia Review tests, 2 EK exams, 1 Kaplan exam, and all 6 AAMC exams. Well, I only wound up taking AAMC III-VI and both EK tests, and I scored a 36-38R. I just didn't have time to take more tests, unless it meant giving up time I needed to effectively learn and review the necessary material and effectively review the practice tests I took. I think you are better off taking 6 practice tests and carefully reviewing each question and especially each wrong answer, than you are just taking 20 tests and barely reviewing your wrong answers. If you buy 20 tests, don't feel the need to take every one of them at the expense of careful review.
Don't get me wrong, If you can take 20 quality practice tests after you've reviewed the science, and you have time to carefully review each test, you'll be better off. But I know people who did tons of practice tests and questions, and did not ever improve. I think they did not spend enough time analyzing their answers and tests.
In any event, good luck!!
