List of Dahlia species
There are 42 accepted species of flowering plants in the genus Dahlia, according to The Plant List. The sectional classification of Dahlia  sensu Sørensen (1969) as updated by Saar et al. (2003) and Hansen (2004) and (2008) is as follows (excluding infraspecific taxa);

Section Pseudodendron

Section Entemophyllon

Section Dahlia

Section Epiphytum

Unresolved
D. pinnata Type (more properly D. × pinnata) Most likely (= D. coccinea × D. sorensenii).
D. mixtecana  (possibly Section Dahlia or Section Entemophyllon)

See also
List of Dahlia cultivars

References

List
Dahlia