In general, early in prep, you should be focusing on and mastering one area at a time. If you learn, say, divisibility concepts, and then apply them by solving dozens of good-quality practice questions, you'll start to notice all kinds of conceptual connections that you probably wouldn't see if you were studying in a non-systematic way. You can't get that kind of practice using practice tests, so I would not advise working through dozens of practice tests if you want to study efficiently and effectively.
There are three main things practice tests are useful for:
- they help test takers get comfortable with the test taking experience (working from a computer screen, etc)
- they are the only way to practice pacing strategy
- they let you assess your level, which can be useful for specific reasons (deciding whether to book a real test, deciding how to apportion study time between Q and V, evaluating whether a new RC strategy is improving your score, etc)
You can use any practice tests to get comfortable with test taking. Some test takers need a lot of that practice, while others do not. But for the other two purposes, pacing practice and score assessment, the only tests you can use are official ones. They're the only ones that will produce realistic scores, and the only ones that will be realistically time-consuming.
Notice that "conceptual improvement" is not in the list above. If you study just by taking lots of practice tests, you likely will improve, but much more slowly than you would by studying in a more focused way. And depending on the coverage of the tests, you could easily miss out on important concepts, and thus not maximize your level.
In general, most test takers don't need to take all that many tests. It varies from person to person a lot, but often test takers don't need any more tests than the official supply available. If you're taking unofficial tests, I'd view them more like problem sets, and would not read all that much into the scores you get.