Is it an adult or a juvenile? That's a big difference, a juvenile can eat many, many crickets in one day. An adult, not so much--they much prefer their veggies. I don't actually believe there is a set number, bearded dragons have such unique personalities and traits it seems to vary from one to the other. If it's an adult I would recommend giving them a variety of greens, including mustard greens, collard greens, turnip greens, dandelions, and the like. Mix in a bit of some "fun" foods every now and then so it sparks your dragons interest. Here's an awesome link to bearded dragon nutrition :
http://www.beautifuldragons.503xtreme.com/Nutrition.html
If you believe that your dragon is not eating enough, make sure to check your temperatures. Heat is needed for a bearded dragon to digest and this is a major problem if it's not hot enough. Heat = digestion. I just feed my dragon as much as he wants 2-3 times a day. This usually, on average, adds up to about 15-20 crickets. Some would say that's too much or that's not enough, but in reality it's up to the bearded dragon.