Our full length practice test questions are drawn from our general pool of 800+ questions, so you can take about 2 full-length tests before you start to see repeats. We usually recommend only taking one and saving the other questions for more targeted practice, but the way you use your question bank is completely up to you. However, you're always free to reset your questions and keep taking practice tests as many times as you like! You can learn more about our tests in this post on our GMAT blog.
If you're looking for additional practice tests, there are a few other options. You can take two free GMATPrep tests from GMAC here, if you haven't already: Download GMATPrep. Next, here's a free test from Manhattan (their materials are great!): Manhattan Full GMAT Practice Test.
Another good way to get an additional practice test from Manhattan is to purchase one of their GMAT Strategy Guides: Manhattan GMAT Strategy Guides. Doing so grants you online access to the first exam in their set of six. However, if you have the desire, time, and funds for a large number of practice tests, a better deal might be to purchase online access to their full set of six exams: Manhattan GMAT Practice Tests.
Hope this helps! :)