The level of classification in biology that includes the most species is the species level itself. In the hierarchical system of biological classification, the species is the most specific level. It groups organisms that are capable of interbreeding and producing fertile offspring.
Above the species level, there are several broader categories, including genus, family, order, class, phylum, kingdom, and domain. Each of these levels groups together multiple species based on shared characteristics. However, the species level is where the greatest diversity is found, as it encompasses all the individual organisms that share a common set of traits and can reproduce with one another.
For example, the species Homo sapiens includes all humans, while the genus Homo includes other extinct human species like Homo neanderthalensis. The family Hominidae includes not only humans but also great apes like gorillas and chimpanzees. As you move up the classification hierarchy, the groups become broader and include more species, but the species level remains the most diverse and specific.