The short answer is about six, and the official ones from mba.com are the ones that matter. GMAC offers six official practice tests (2 free and 4 paid), and each can be reset once for a fresh set of questions, so you effectively have twelve available if you want them. Six is plenty for most people.
That said, the number of tests matters less than how and when you use them. One practice test belongs at the very start, as a baseline, to tell you roughly how far you are from your goal. After that, taking more tests isn't a good use of your time until you've actually learned the material. Practice tests don't teach content, they measure it. Taking them before your fundamentals are solid just gives you a low score and burns through a limited resource.
The real practice-test phase comes at the back end of your prep, once you've worked through the content. In that stretch, take one official test roughly every week and finish them all by the week before your exam. The reason for spacing them out is the part most people underestimate: the value isn't in taking the test, it's in the review afterward. After each mock, go through every question you missed and every one you guessed on, figure out why it happened, and go back to fix the underlying gap before the next test. A test you don't fully review is a test you mostly wasted.
Use those later mocks to do three things: confirm your pacing under real conditions, test your section order so it's locked in before test day, and build the stamina to stay sharp across the full exam. If you're still deciding on section order, try your chosen order on two or three of them before committing.
As for scheduling the real thing, the cleanest approach is to wait until you've hit your target score on at least two practice tests before you book your date. That way the official exam is a confirmation of what you've already shown you can do, rather than a roll of the dice.
This article covers practice test strategy in more detail, including timing and how to get the most out of each one:
GMAT Practice Test StrategyPurvaG
I've heard everything from 2 to 6. Wondering what the sweet spot is without burning through all the official material too early.