Stomata, are the mouths of plants. Plants breathe through them, just as we do with our lungs. They are not visible to the naked eye, and a plant therefore needs quite a lot of them. However, there must besome space between the stomata, otherwise they cannot open and close properly. How many stomata a plant makes is dependent on the presence of the protein Too Many Mouths (TMM, 'Too Many Mouths’) This protein is so named because a plant is going to make too many stomata in the absence thereof.
Stomata are not only the "lung" of plants, they also play an important role in the evaporation of moisture. When stomata are open, a plant evaporates lots of water. Plants have to be careful that they do not make too little, but also not too many stomata. If they make too little then they cannot bring in enough carbon dioxide for photosynthesis. And if they have too much of them, they run the risk that they will dry out quickly when opening up their mouths.