Why is there no fixed number of organisms that must belong to a taxon?

There is no fixed number of organisms required to belong to a taxon because biological classification is based on recognizing natural groups through their resemblances and differences, rather than adhering to rigid quantitative thresholds.1

Taxonomists aim to place organisms into groups that reflect their evolutionary relationships and shared characteristics. Because these groups are defined by their biological affinities rather than a set number of members, a taxon may contain a single species or thousands. For instance, if a newly discovered organism is sufficiently distinct from all known forms, it may require the naming of a new genus, family, or even a higher-level taxon, regardless of whether it consists of only one species.21

Furthermore, there are no explicit, universal taxonomic characters that define the rank of a group (such as a phylum or class). A feature that serves as a diagnostic trait for a high-ranking group in one lineage may be highly variable within a lower-ranking group in another. Consequently, the assignment of rank and the grouping of organisms rely on the consensus of practicing taxonomists, which remains subject to revision as new information about evolution and genetics is discovered.12