It all depends. Classification is an every evolving subject. A fish gets classified as a Danio species one year, 20 years later, it is found to be the same as another species of Danio, 20 years later it is found to belong to a different genus altogether, (in the case of Danio the genus Devario is an important division).
There are over 50 species listed with Danio as the genus, not all of them are valid, and the removal of Devario sp. reduced the number considerably. There are, however, more than 15 species of Danio still recognised hence I voted 15+.