According to Wikipedia, there are a whopping 59 metal subgenres, and that's just on one page. Now, I think that we can lower this number. Let's start by getting rid of these picky-picky subsubgenres. Why do these exist? Goodbye "Blackgaze, Death n' roll, Pagan Metal, and Drone Metal", along with several others. Now, we're down to 26 subgenres. Sounds good, but it can still be narrowed down.
First, let's narrow it down further. We will combine Alternative Metal, Avant-Garde Metal, Kawaii Metal, and Progressive metal into a separate category, nerd metal.
The next category will be called "metalhead metal." It will consist of the sort of metal obnoxious metalheads act like is the only TRUE metal. Metalhead metal will be made up of Black Metal, Death Metal, Doom Metal, Extreme Metal, Grindcore, Power Metal, and Classic Metal.
Next up, we have homeschool metal. Homeschool Metal is just Christian Metal and Folk Metal.
The fourth subgenre will be Who Cares Metal. Who cares metal is all the claimed subgenres that don't fit into one of these other ones but don't fit in with each other, either. It includes Crust Punk, Glam Metal, Industrial Metal, as well as some other stuff. It is the Hufflepuff of Metal subgenres.
However, we can narrow it down more.
Metalhead Metal and Who Cares Metal will be combined into "Metal your parents get angry about you listening to because you will become a Nazi-Commie, get a face tattoo, join a gang, and die of gunshot wounds."
Nerd Metal and Homeschool Metal will be combined into "Metal your parents get angry about you listening to because you will become a social outcast, get a "Star Trek" box set, join a fellowship, and die of a +4 Vorpal Sword.
These two can be combined into metal, a subgenre of rock categorized by powerful lyrics and fast playing.