What is study of population statistics called as ?

Demography

The answer is Demography.

Demography is the statistical study of human populations, especially with reference to size and density, distribution, and vital statistics (births, marriages, deaths, etc.). It is a social science that seeks to understand and explain patterns of population change. Demographers use a variety of methods to collect and analyze data, including censuses, surveys, and vital statistics.

Geography is the study of the physical features of Earth and its atmosphere, as well as the distribution of people, plants, and animals. Genealogy is the study of family lineages. Histology is the study of tissues.