Are there many non-biracial mixed race people?

Why do you find it annoying? lol

Because I literally never find non-biracial mixed people, if I search mixed race, they are always biracial. I go on the mixed race subreddit and it is all catered to biracial people (and even then mainly half white half other ethnicity), I don't have a problem with people like that, I have family like that and so on. But it seems the mixed race narrative is for biracial people, a lot of blog posts all preach a similar message.

What's the difference and why the interest with it?

The difference is your parents could be mixed race as well and not the same mix as each other which makes you not biracial.

Because it is assumed that if you are mixed race you are biracial, which isn't always the case. I understand it is maybe less likely but I feel I've never met anyone who shares this with me (other than my own family I guess or people from my country but I don't see them at all). The interest is

Wait a generation or two, biracial people will be having kids together

I'm half Japanese and half Irish (sorry!) so my children will undoubtedly be mixed race