I'm not trying to dissuade you from using a mouth guard, but you might want to talk to a doctor or dentist first. I lost two teeth from a baseball and the teeth were hit with so much force that they came out very cleanly. Now he is no physicist, but he told me that if I had been wearing a mouth guard, I would have lost more than just two teeth.
In layman's terms (since I am also no physicist), a mouth guard redistributes the force across a wider area in your mouth when hit by a baseball. So when I got hit straight on and lost two teeth, the redistribution could have caused me to lose more teeth. Since I would assume the most likely way you are going to get hit is straight on (comeback from as a pitcher or bad hop as an infielder), a mouth guard may not be the right choice.