Is John the sole part/leader of the Hunger, or are there multiple other entities in there?
This is still unknown. It’s true that he helped create the Hunger into what it is now - a living plane with the goal to absorb every material plane, fueled by dissatisfaction and discontent - but whether or not he’s the leader is yet to be seen.
As for being the only entity within the Hunger; no, he’s not.
John: […] And I suppose when I say we, I’m talking about everyone in here. Everyone– it’s weird, Merle. I’ve never talked to anybody about this outside. Usually my transactions with other people are adding them to my being, so I don’t–
Griffin: It’s been a couple dozen times. He has a chessboard set up. It seems like he can kind of make things in this space to make himself sort of more comfortable, and so you talk to him, and you’re– obviously there’s animosity between the two of you, right? Because he’s been killing you all these times, and he is this supernatural force that is pursuing you, but he sort of lets on that this is also the only time that he ever talks to anybody or exists as John. And so he kind of looks forward to it a little bit. He tells you that.
John: […] You’re the first person who I’ve, sort of, talked about this to who hasn’t listened. There were– everyone listened, Merle. I’m not being hyperbolic. Every. Person. In the world was swayed. I don’t know why you’re different. But everyone else listened. Everything, everyone across our whole plane of existence- our- our shared vexation, with, life? Covered the world like a blanket. And soon every bird in the sky and every tree in every forest and every blade of grass, and grain of sand shared our fury. And it- it wasn’t long before… it changed us.
Griffin: And I think as he’s talking Merle, you see this scene outside, this constant orange sunset, start to turn inky and black, with these colorful ribbons of light that you’ve seen inside the Hunger so many times. And he says,
John: We changed our entire plane into something new altogether. A single being, fueled by… discontentment. Searching for something bigger than this existence, regardless of the cost.
Griffin: He turns towards you and he says,
John: You call us The Hunger. That’s not entirely inaccurate. ‘Cause we are… hungry. But it would be more accurate to simply call us… Dissatisfaction. But soon–
Griffin: And he holds up his hand, and says,
John: –you will call us… Ascendant.
John’s not the only one in there; he’s just the only one who talks with Merle.