Its actually 4 including the latest one. You can find more details about the old ones here. Same link for downloading them.gmat-prep-power-prep-official-free-gmat-tests-77548.html
For the latest, you can get it from mba.com.
There is not huge difference in the question bank, 90% of the questions will be common, but you can find some new questions every time you repeat the tests. If you haven't taken any GMATPrep tests
so far then use the old version test two times and the new version two times. 8 tests are more than enough. Repeat test scores will be inflated.
KUDOS - if my post has helped you.