The level of classification that contains the least number of organisms is the species level.
In biological classification, organisms are grouped into hierarchical categories. These categories include, from broadest to most specific: domain, kingdom, phylum, class, order, family, genus, and finally, species. As we move down this hierarchy, each level narrows down the number of organisms included.
The species level is the most specific category, representing individual organisms that can interbreed and produce fertile offspring. For example, within the genus Canis, there are several species like Canis lupus (wolves) and Canis familiaris (domestic dogs). Each of these species contains a much smaller number of organisms compared to broader categories like domain or kingdom, which encompass a wide variety of life forms. Therefore, species is the classification level that holds the least number of organisms.