Abnormalities of tooth number include hypodontia and hyperdontia. Hypodontia is the most common dentofacial anomaly in humans and is defined as the agenesis of one or more teeth. The term hypodontia refers to the absence of up to 6 teeth. Oligodontia indicates the agenesis of 6 or more teeth, while anodontia refers to the complete absence of teeth. Hypodontia arises as a result of complex interactions between external and genetic factors and can be nonsyndromic or isolated, as well as syndromic. It is inherited as an autosomal dominant, autosomal recessive, X-linked, or polygenic trait. Along with hypodontia, other anomalies such as microdontia, conical teeth, and taurodontism often occur. Hypodontia can be one of the symptoms of a large number of general genetic disorders. The form of syndromic hypodontia can be specific or nonspecific. Dentists play a significant role here, as they are often the first doctors to suspect a specific syndrome, enabling early diagnosis and timely initiation of treatment. Another tooth number anomaly associated with genetic disorders is hyperdontia, or the presence of supernumerary teeth. The purpose of this study is to provide an overview of the current literature on genetic abnormalities of tooth number and their association with some of the most common syndromes. Furthermore, the association between hypodontia and colorectal cancer and ovarian cancer will be discussed, as well as the possibility that the hypodontia phenotype is an early indicator for later neoplasms. A case of a patient with hypodontia and a genetic disorder will also be presented.