Leafly lists 6,822 cannabis strains. Though ‘strains’ or more aptly cultivars are often not reflective of the actual underlying chemical nature of the plant [source]. They are ultimately not very meaningful in general.
Based on this database, Europe has 104 varieties of hemp registered.
Though potatoes and cannabis aren’t really a fair comparison for many reasons. There are a lot of different species of potatoes with major chromosomal differences but cannabis only has 3 species.