How many genders are there ?

Humans have 46 chromosomes, from which 22 pairs are autosomes and one pair of sexual chromosomes. Because of their shape, sexual chromosomes are named X or Y. If a person has XY chromoses that persone is a male - you probably know that from basic biology. Y chomosome caries specific genes called SRY who promote testes development and inhibit development of female reproductive apparatus. Absence of SRY makes a person female, for example if a person has XX or even lacks one sexual chromosome.
In my opinion everything else associated with so called multiple genders is a behavior that a person chooses based on knowledge and experience from his or her surroundings.