The genus Adenium consists of 11 phenotypically recognizable types in cultivation. There are at least 8 good species in nature, and perhaps several more (see map below). For ease of labeling, this site treats each type as a separate species, and I will use the names that are popular among horticulturists until the taxonomic confusion has been sorted out. Specific epithets (the second word in a binomial [scientific] species name) that have not been validly published and incorrect published names are enclosed in double quotes. (Very important: single quotation marks identify a cultivar name.)
The taxonomy page summarizes what is known about adenium species, and the nomenclatural issues page summarizes the problems. See Dimmitt and Edwards 2021 for a detailed analysis.
* 2021. Dimmitt, Mark A. and Taylor Edwards. Adenium Taxonomy and Nomenclature: Progress Creates More Questions. Cactus and Succulent Journal (U.S.). Vol. 3 no. 4, pp 252-266. Available as a PDF on request to the first author.