The correct term for upper incisors should be maxillary incisors, there should be two maxillary incisors under normal circumstances, it is not normal to grow only one, the possible causes are blocked incisors or congenitally missing incisors, as follows.
1. Blocked incisors: Insufficient space for the eruption of the incisors or the existence of soft tissue obstruction will lead to the blockage of the incisors, resulting in the growth of only one maxillary incisor. Medical intervention can be done to open up the space or cut the gums to help the blocked incisors to erupt.
2. Congenital absence of the incisors: X-ray examination confirms that there is no tooth germ of the incisors in the jawbone, i.e. the incisors are congenitally absent, and there will not be any incisors erupting in the future, so the gap can be opened up through orthodontics, and then the missing teeth can be restored through implantation.