It really depends on the size of the mountain and the size of the engine (your ability as a climber). And are you really going to be just carrying 20 lbs?
If you're going to have any kind of long climbs (more than a few kilometers), in my humble opinion, you should get a triple chainring (AND a cassette that goes to 27). I consider myself a decent climber and I really like to have my triple when I go climb hills in North Vancouver (the Cypress hill climb is 11 km long but I use the triple a lot more than just there). Your legs and especially your knees will thank you for being able to spin, especially on multi-day trips.