A normal person will usually have two sets of teeth in his/her life, milk teeth and permanent teeth. There are 20 milk teeth in childhood, while permanent teeth in adults are usually between 28 and 32. 1. There are 20 milk teeth in childhood, 5 on each side of the upper and lower jaws. Usually the first baby tooth will start to erupt from about 6 to 8 months after birth, and all the baby teeth will usually erupt around 2.5 years old. 2. Adults usually have 28 to 32 permanent teeth, about 7 to 8 on the left side of the upper jaw and 7 to 8 on the right side of the lower jaw. Usually, the first permanent teeth start to replace the milk teeth or erupt around 6 years old, and the replacement or eruption is completed around 12 years old. If you have an abnormal number of teeth or if you have dental or periodontal disease, please go to the hospital for a checkup, and follow the doctor’s instructions for treatment based on the results of the checkup.