In general, if you want to ride your mountain bike at night, you will need a light of at least 500 lumens on your handlebars, plus another 500 on your helmet. If you only want a light on your handlebars, at least 2000 lumens will provide plenty of illumination. There are a number of different factors which impact this figure including your night vision, riding skills, terrain, speed, beam pattern, and the weather conditions. For example, if you are a beginner riding on unfamiliar terrain, the extra lumens will certainly be worthwhile.