Usually, taking about 7-8 mock GMAT exams before actually attempting the real GMAT is good enough for practice and strategy fine-tuning. Often candidates believe that the more the number of tests they solve, the better prepared they will become. This need not be true. What is more important than the number of tests is the quality of the tests as well as what you learn (identifying areas of improvement) from each test you take.
At Sharp Minds, we strongly advise all our students to devote quality time analyzing their performance on each test they take. This time can easily be between 2 and 5 hours per mock GMAT exam.