Do your own research?

Fun fact: Modern science does not say that there are only two biological genders but that even on a biological level, there is a spectrum. For example, there are measurable differences between the brains of a woman and a transgender man and there are more viable chromosome combinations than XY and XX.

Heart's in the right place but can I just break down "biological gender" for a sec? Gender is more or less a Victorian euphemism for "sex". The word's original usage and origins are actually close to "genre" like book genre and in modern usage it works closer to "genre" as it describes an internal sense of affiliation to a cultural or phenotypic category or abstention from category drawn culturally broadly along reproductive groups. It's a fuzzy line that we don't use biology to try and police on purpose. It's more the realm of psychology and wrapped up in questions of what supports a fulfilling life of interpersonal connection.
Gender is a lot of things but if you are talking about sexual characteristics and intersex people it's important to distinguish and highlight that discussions about sex characteristics and gender distinctions are two separate conversations with two mostly unrelated minorites. Very often I see intersex people's existence being used as a conservative gotcha for support of trans folk like myself and... It doesn't work. They know trans and intersex folk aren't the same thing and you are trying to red herring them off of the discussion they really want to have. Also... Intersex issues deserve more individual spotlight where they aren't mashed in with transgender advocacy. They have their own unique battles with beaurcracy, society, mental health and so on. Give em some love sometimes. Talk about how our social concepts of binary sex has medical and social consequences for them. They need the press too.
Severing the old euphemism "gender" completely from the word "sex" so we can better have these discussions is a start. A lot of Conservatives use the two interchangeably to imply that gender doesn't exist, to them it is just a synonym for sex so word combos like "biological gender" is developing into a unique alt right dog whistle with it's own google alt-right web result pit so just wanna give you a heads up on that front.
- I’m aware there’s a distinction between gender identity (psychology), gender expression (sociology) and sex (biology). I used the "biological gender" term since the comment I’m answering to did. I could have expressed myself clearer though. I’m just gonna use the fact English isn’t my mother tongue as an excuse here…
  Generally my argument is „sex isn‘t binary (and shouldn’t be regarded as such), so why would gender (both identity and expression) be?“
  Besides, sex is a spectrum beyond chromosomes. Even two XX or two XY people can have differing physiologies when it comes to hormones, etc. resulting in one appearing more masculine and one more feminine.
  
  It's rough even for English as a first language speakers. The alt-right, conservatives and transphobes in general have these little subtle words they use to signal their transphobia to each other and signal their intent to troll or that their real intentions while pretending to debate in earnest. "Biological gender" is a more frequent one but they get waaaaay more subtle.
  One you see in the UK that is SUPER subtle is transwoman or transman. Properly speaking trans is a separate adjective so it modifies the noun. "The woman is trans" in effect. By removing the space in trans woman they sort of signal their intent and allegiance by creating a whole new noun as in : "That's not a woman that's a transwoman" it's so subtle it could just be accidental and leaves plausible deniabillity for the user of the term... but trans folk get used to seeing these little cues that signal that the writer or speaker is getting their talking points from very specific hostile sources. Language has been very weirdly weaponized but when you understand what is happening it can help you figure out the diehard transphobes who are wasting your time and good faith from the people actually just properly duped by the misinformation brokered by those with malicious intent.
Klinefelter syndrome with XXY is the most common of the non-standard combinations, but only account for about 1/1000 of male births. Combinatorics of sex chromosome aneuploidies such as XXX, XXY, XXXY and so on does only give a finite number of combinations.
I do not really see why this argument is brought up so often. Is it neccesary to speak of chromosmes to validate gender identity?
- It is basically the entire premise of their argument, and it's wrong, so yes.
- Since most transphobes base their arguments primarily on a perceived biological binary, the argument needs to be brought up.
  The psychological and sociological arguments around gender identity and expression sadly aren’t very fruitful when the people you’re trying to convince fixate on biology knowledge that has been refuted for decades.
  
  Okay, I see. I am not from the US, which might be guessed from my misspelling of "necessary" above. I am sorry you have to deal with those people, but for what it is worth they are not so abundant here (in my experience at least).
  
  I’m not from the US either, have met my fair share of idiots both on- and offline though…
  
  Did not mean to make an assumption about you, merely say that the culture war waged by reactionaries is foreign to me.
  
  No worries. I’m luckily also mostly spared from personal confrontations with assholes in my country
- You have to connect with people based on their existing world view. If they think gender identity must be exclusively determined by chromosomes, then talking about XXY and YYX and whatnot can help them see that it's not as simple as only two genders. It's my understanding that the intersex rate is more like 1%, but it's been a while since I looked into it and I don't know the mechanism behind the other ones.
  
  It could very well be higher than I wrote as there is evidence of it being underdiagnosed. I just do not see why chromosomes are pertinent to discussing gender identity when I think the point is that anyone should be able to self determine whatever they want to identify as.

Basic biology: there are 2 genders

Basic math: 15 is not divideable by 4

Mathematical biology: the amount of genders is not a natural number (assuming it's a spectrum), but it is countable
- Never heard of π-sexuality?
  
  Hey we’ve all watched American Pie.
  
  Pisexuals should just stop kidding themselves
  \frac{ing}{idiots}
  
  But seriously: isn't there a bride flag with a π on it?
  Edit: it's polyamory

biological gender

Is that something like clockwork joke or liquid ennui?

Because of the fact that it takes roughly 100,000,000 years for DNA to double in complexity, some scientists have estimated that we actually are closer to 7 billion years of necessary evolution to create us. That just raises more questions though since our planet is around 4.5 billion years old.