The Population of the Abyss?

For sake of calculation let’s say there are only the 666 layers of the abyss, how would you go about calculating how many demons inhabit that space? I’m trying to set up a campaign where the party’s mission is to kill all demons and they want a big number

EDIT: Okay so I understand how the abyss works, I’m not new to D&D lore. I literally just want to know how to get a gauge for population of this many layers without the answer, “it’s endless”. I know it’s endless, but I’m speaking theoretical.