Hi arif24,
There's no 'minimum' number of practice CATs that you should plan to take during your studies, but you should certainly plan to take at least a few (at regular intervals) so that you can track your progress and learn more about your 'weaknesses' - so that you can focus on the proper areas as your studies progress.
Before I can offer you any additional advice, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied?
2) What materials have you used?
3) How have you scored on each of your CATs (including the Quant and Verbal Scaled Scores)?
Goals:
4) What is your goal score?
5) When are you planning to take the GMAT?
6) When are you planning to apply to Business School?
7) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich