The classical outfit of a black robe and scythe would be more like a uniform, and there would have to be millions of Grim Reapers to harvest souls when humans die, since millions die daily.

And, since the global population grows every years, their numbers would have to grow as well.
How do you think the Grim Reaper species increases its numbers?
Do they procreate?
Or do they recruit new members of theirs species, like this?