An endangered species is defined as a species that's "in danger of extinction throughout all or a significant portion of its range [the geographic area in which a kind of animal or plant normally lives or grows]." The determination of which species are considered "endangered" is a complex process.
While the population size of a species is a factor, there is no set "number of living members" below which a species is defined as endangered. For instance, a species with a million living members, all of which are clustered in one small area, could be considered endangered; whereas another species having a smaller number of members, but spread across a broad area, might not be considered endangered. Another factor is a species' reproduction data, such as the frequency of reproduction, the average number of offspring, and their survival rate.