I normally do two on flatter ground because I get a rythm and I'm a four legged creature.

I often switch to one in rough terrain (like a mountainside) so
I have a hand to put down if I'm semi-scrambling.

In that case, my pole is on the *uphill* side. because If I lose my balance I want to lean into the pole and toward the safe side, not the side I'll pitch over..