How can two individuals of the same species have different chromosome numbers?
there are diverent concepts of definitions of species:


morphological species concept, distiguishes a species by body shape and other structural features
ecological species concept, distighuishes a species according to it's niche
phylogenetic species concept, smallest group which schare a common ancestor
biological species concept, a species is a group whose members can produce fertile offspring


the biological species concept is most commonly used, if we apply this on your problem, they still produce fertile offspring, so they are the same species.

If for example a mutation wil occur which causes polyploidy (more likely in plants, however it's possible in some animals). Let say the orginal species have 2n = 12, if polyploidy occurs(during meiotic error for example). We will have 2n = 24. If these species produce gametes n = 6 and n =12, fusion of those gametes will result in unpaired chromsomes, which in turn will lead to a reproductive barrière. The 2n =12 and the 2n = 24 species can only mate with each other to produce fertile offspring, so we can say these are different species.