There isn't a single answer to the question for all students—but I would say most people will take between 3 and 6.
CAT 1: Very early on, within the first week or two. You won't like your score, but the point of this early one is to get a sense of your natural strengths and weaknesses. Use that info to help prioritize your studies.
CAT 2: About halfway through what I call your "primary" studies—your first time through your major study material. You still won't like your score—and you're even more likely to mess up the timing on CAT 2 than on CAT 1—but you need to start getting some practice in putting together all of the various things you're learning. And again, see which of your natural weaknesses have gotten better via your studies and which still need more work.
CAT 3: Towards the end or at the end of your primary studies. Take stock / see where you're at in terms of both score and overall strengths and weaknesses. Use this data to help figure out the lowest-hanging fruit—the things that you have the best opportunity to improve via review and practice.
The above assumes you're going for a 700+ kind of score and/or you're going for a 150+ point improvement. If you don't need to see that kind of lift, then you may compress the study cycle and only do CATs 1 and 3 listed above.
Then, do a series of 2-3 week cycles, with CAT 3 as the first CAT in the first cycle:
– Take a CAT
– Analyze and fill your Buckets; this article explains:
https://www.manhattanprep.com/gmat/blog/2014/02/11/4-steps-to-get-the-most-out-of-your-cats-part-1/*Note: If you have access to our Atlas study platform, you can also find videos that explain how to do your CAT analysis, along with a spreadsheet template to use to help analyze your data and draw conclusions.
– Spend the next 2-3 weeks focused on improving Bucket 2 items
– Repeat the cycle. Each time you take a new CAT, you'll see how close you are to your goal score; when you get within range, schedule the real thing.
Most people will do that cycle at least once; the exact number will depend upon your goal score and how far you have to go to reach it.
When you're ready to schedule the real thing, you may take 1 or 2 more CATs to solidify your game plan, as described here:
https://www.manhattanprep.com/gmat/blog/2014/11/07/last-14-days-gmat-part-1-building-game-plan/(This is the stage to use the official practice tests from GMAC, by the way.)