The concept you are referring to is often related to the "minimum viable population" (MVP) size, which is the smallest population size at which a species is able to sustain its numbers and avoid extinction. This number can vary greatly depending on the species, its reproductive rate, genetic diversity, environmental factors, and threats it faces.
In conservation biology, a more specific term often used is "critical mass," which can refer to the minimum number of individuals needed to maintain a viable population. However, "critical mass" is more commonly associated with nuclear reactions and less so in the context of species conservation.
When a species falls below a certain population size, it can be classified as an "endangered species," potentially leading to extinction if conservation measures are not implemented.
In summary, while there isn't a one-size-fits-all number, conservationists generally aim for specific population thresholds (e.g., hundreds or thousands of individuals) to ensure a species' long-term survival.