Genitalia-based categorizations:
1: People who have penises are male, and people who have vaginas are female.
Chromosome-based categorizations:
1: People with XX chromosomes are females, people with XY chromosomes are male.
Reproduction/fertility-based categorizations:
1: People who can impregnate females are males, people who can be impregnated by males are females, and people who can neither impregnate nor be impregnated (e.g. pre-pubescent children, sterile people, old "women") are genderless.
Self-identification based categorizations:
1: People who self-identify as male are males, people who self-identify as females are females. There are only two genders, "non-binary" is not a thing.
2: Same as #1 but non-binary is a thing, so there are three possible genders.
3: Same as the above but gender is a spectrum rather than having a definite number.
There could be many different categorizations:
-One gender (?): humans
-Two genders: male and female, defined mostly by genitals
-Two genders: male and female, defined by self-identification (think transgender culture, except without non-binary)
-Three genders: children before puberty (considered 'genderless' in this categorization), adult males, adult females
-Three genders: males & females (includes children, and adult males and females who are capable of reproduction), adults incapable of reproduction
-Four genders: children before puberty, adult males & females (capable of reproduction), adults who are incapable of reproduction
-Five genders: children before puberty, adult males & females (capable of reproduction), adult males who are incapable of reproduction, adult females who are incapable of reproduction
-Six genders: male children before puberty, female children before puberty, adult males who can reproduce, adult females who can reproduce, adult males who can't reproduce, adult females who can't reproduce
-Three genders: anyone capable of producing sperm that can fecundate an egg ('males'), anyone capable of having an egg that can be fecundated by sperm ('females'), anyone incapable of reproducing (children, sterile adults, old women and others) ('genderless')
-Three genders: male, female, non-binary, and is defined by self-identity (very similar to many forms of modern transgenderism)
-And many, many others.