Though not explicitly stated, species designations are mostly consistent with being incremental, i.e. each newly encountered species receives a number one higher than the previous one. Some anomalies exist, such as the Alpha Quadrant Ferengi having the low number of 180, which may hint at an early first contact with the Borg under unusual circumstances. Numbers do not appear to be re-used even after the species has been assimilated. Whether the Borg have a designation for themselves is unknown.
Several additional species designations are given in non-canon works: