Agathys

Actually, according to Planes of Conflict, each layer is a string of orbs. The deeper the layer, the smaller each orb and thus the farther away from the two surrounding orbs. The deepest layer, Agathys, has orbs so small and far away that they cannot be seen from adjacent orbs.

On every layer other than Agathys, other orbs are visible in the sky. The further "downward" you travel in the nested layers, the smaller and more distant the orbs seem to be.

In Agathys, the orbs are so small and far apart that each seems to be alone. This is mentioned in both Planes of Conflict and the 1e Manual of the Planes, I believe.

I'm not sure why the 3e Manual of the Planes decided there was only one orb in Agathys, but you could rationalize that they meant that only one was visible. It's unlikely the players will ever reach another one, in any case.

With teleportation and portals, it's very easy to get them to another orb if you want them to get there. I just meant that they're unlikely to get there on their own. It's hard to teleport to a place you've never been to, especially if you can't even see it in the sky.

Possibly. Like I said, it's certainly not impossible to get to another orb if it's important for them to do so. I just think it's unlikely any players will even think of doing so on their own, considering it's invisible and billions of miles away.

If they know for a fact that there's something on another orb that they want, there are all sorts of options available: scrying, portals, teleportation, traveling the void on flying ships.