Should I learn one meditation technique or try several?

One, for long enough that it becomes familiar. Sampling many techniques gives you a passing acquaintance with each and fluency in none, and most of the benefit arrives after a practice stops feeling novel. Pick the one that suits how your mind works, breath focus for simplicity or body scan if you prefer a clear next step, and stay with it. In StillMind a Techniques course is built around exactly this: one technique carries the whole course, opened up a piece at a time, so you get properly good at it rather than half-knowing five.