According to the AviList checklist v2025 (link), the taxon comprises 75 species in 19 genera and three families. 

Genus-level timetree of extant Bucerotiformes based on Gonzalez et al. (2013) and Tamashiro et al. (2019). The distribution of each genus is indicated by the colour-code used throughout this website (Distribution code). Interfamiliar divergence times have been adopted from Stiller et al. (2024), who considered four bucerotiform genera (Buceros, Bucorvus, Rhinopomastus and Upupa). Intrafamilar divergence times have not been determined yet.

Traditional genus-level classification of extant and recently extinct (red letters) Bucerotiformes, following AviList checklist v2025. (link) The number of subspecies is given in parentheses.

References

Gonzalez JCT, Sheldon BC, Collar NJ, and Tobias JA (2013), A comprehensive molecular phylogeny for the hornbills (Aves: Bucerotidae), Mol. Phylogenet. Evol. 67, 468-483. (abstract) (corrigendum)

Kuhl H, Frankl-Vilches C, Bakker A, Mayr G, Nikolaus G, Boerno ST, Klages S, Timmermann B, and Gahr M (2021), An unbiased molecular approach using 3'UTRs resolves the avian family-level tree of life, Mol. Biol. Evol. 38, 108-127. (free pdf)

Tamashiro RA, White NE, Braun MJ, Faircloth BC, Braun EL, and Kimball RE (2019), What are the roles of taxon sampling and model fit in tests of cyto-nuclear discordance using avian mitogenomic data?, Mol. Phylogenet. Evol. 130, 132-144. (abstract)