My schedule for doing them was:
NBME 13: 12 weeks out.
UWSA1: 10 weeks out.
NBME 18: 4 weeks out.
NBME 19: 2 weeks out.
UWSA2: 1 week out.
Free 120: 4 days out.
In addition to those I had to take 4 practice tests for COMLEX for my school; March, May, and June, and July which is why I took the ones I bought at random intervals.
I would invest in as many as it takes for you to get comfortable with testing conditions; Private space, do them timed, no access to outside resources for your eyes to wander to, etc. That depends on you though. It might be 1, it might be more. I felt like I did just enough. As for which ones, I can’t say since I only bought the 3.
I took Step 1 today and I felt like it was worth it, plus I only spent $180, since UWorld is basically mandatory and it’s only $10 more to get the second assessment if you don’t get the long subscription.