noun
population (of animals, etc.); population size; number of individuals
Used especially in ecology and biology for the count of individual organisms in a population. More specific than 人口 (human population).
この地域のシカの個体数は年々増加している。
The deer population in this area is increasing year by year.
絶滅危惧種の個体数を調査する。
Survey the population size of endangered species.
頭数 is a more general term for the number of animals (often livestock), while 個体数 is a technical term used in ecology.
Compound of 個体 (individual, organism) and 数 (number).