I don't think there's really a magic number of tests you should take. If you feel that you grasp the material well, do more practice tests. However, if you don't know the material very well, your time might be better spent trying to get a better understanding of material you are weak on. I would definitely second the above posters recommendation to buy the AAMC practice tests. I probably took about 15 practice tests (most of them I skipped the writing sections) and got a 30.