Well, the answer is as many as possible, but it will depend on your schedule and time constraints. Taking one or two practice exams is obviously better than none. But in general, the more practice tests you take, the better prepared you will be when you take the actual CMA exam. If possible, take several practice tests early on in your studies so that you can identify knowledge gaps and address them in your overall study plan.
If you have time, try to take at least 6-10 full-length practice exams. That may seem like a lot, but research shows that the more practice you get, the faster and better you will become at answering questions correctly, which is the key to passing the exam. Most quality test review companies give students access to a test bank with a few thousand questions, providing the opportunity for an unlimited number of practice exams in a simulated test environment.