Our full length practice test questions are drawn from our pool of 900+ questions and you can take about 6 full-length tests before running out of new questions; however we recommend only taking 2-3 and saving the other questions for more targeted practice. You can also reset your questions and retake tests over and over again. You can learn more about our tests here: http://magoosh.com/gre/2012/full-length-gre-practice-tests/
To access practice tests, you can click on the link on the dashboard that says "Take a practice test"-- it's in the Quick Practice section, right above the green Practice Verbal buttons.
We also recommend Powerprep from ETS: http://www.ets.org/gre/revised_general/prepare/powerprep2