Ideally you should take as many as possible. The important thing though is that you're getting something out of each PrepTest. So don't just do 3 tests a day until you burn through them all (extreme, I know). J.Y. recommends doing a least one test every other week (min.) up to three tests per week (max.). To answer your other question, you should start testing after building a solid foundation of understanding (not a specific date from your LSAT administration date).
As many Pt as it takes to get the score you want. I don't think you should wait to start studying. Study now and take the test when you're ready. Apply to LS soon after that.
Comments
Hope this helps!