It all depends on the intensity (wattage) of the light. Most UFO lights (circular with fans on the back - so they look like a ufo) are 90 or 120 watts. A 90 watt LED grow should be used for 1 plant. If you have a 120 watt LED grow light, you will be able to grow 1-2 plants. These lights should be hung 16-18 inches from the top of the canopy of your plants. For larger, more intense lights the distance will increase. http://www.dormgrow.com/proper-distance
Plants reach for the light source, just like they reach towards the sun. If your light is too close it will stunt growth.