I think that, depending on your progress and your target score, 4-6 tests would be sufficient. Adjust as you need to. One a week is fine. I read early on that doing practice questions, untimed, on weeknights, followed by full, timed tests on weekends, was the way to go. Get into a groove with one question type at a time, then move on to the next.
As far as I know, the only practice tests that are adaptive are GMATPrep, Powerprep, Princeton Review
and Manhattan GMAT
. Of those, GMATPrep is most essential Powerprep is old, but still useful. I would do the PR tests before doing the Powerprep tests. Very much like GMATPrep. MGMAT is becoming notorious for overly complicated quant questions, but if you don't freak out at that, then they'd still be worth doing if you want additional tests.