Is there an advantage to more people? Just to have more?
I think so.
Consider a single molecule in a box. It bounces around. You can sort of predict it. Not that interesting.
Get 10^23 molecules in a box, and you get emergent phenomena, like sound waves and turbulence, which you probably wouldn't predict from watching one molecule. A higher level of stuff going on.
Get a bunch of those, and you get single cells. They have another layer of emergent phenomena. Reproduction, metabolism, etc., which isn't predictable from looking at sound waves and turbulence.
Get a trillion cells, and you get humans and human brains. Sentience, thought, invention, emotion. Things you wouldn't necessarily predict looking at an individual cell.
Get a billion humans, and you get a world of nations, with technologies, art, etc. not imagined by any one previous human. A new layer of emergence.
What happens if you get a million or billion worlds? Do we get another layer of emergent phenomena not imagined by one world today?