This is partly a tongue-in-cheek question, but I've just realized that I have five black coats: an oilcloth raincoat, a patterned Desigual pirate coat, a hooded knee-length toggle coat, a Buffy the vampire slayer leather duster, and now a new classic trench. How many black coats is too many? I definitely wanted a black trench, not a beige or navy one. And I'm still on the hunt for the perfect black (or possibly dark gray) winter wool coat.
I do wear all the coats. Not so much the leather duster, because there's only so many times you want to look like a vampire slayer. But in general, if there is more than one colour option for a coat, I always seem to want the black one.