In terms of instancing, there are the following separations and you can't instance with someone in any of the others:
- Odyssey PC Live
- Horizons PC Live
- Horizons PC Legacy
- Horizons XBox Legacy
- Horizons PS4 Legacy
"Legacy" and "Live" are additionally two entirely separate game universes: you can't even interact indirectly with players in those, and even your pilot is separate too (at the moment, if you haven't played for four years, they'll be identical - but the moment you do something in either they'll start to diverge)
If you're on PC then Live has far more players than Legacy (probably about 100x); my friends list suggests that most of them are in Odyssey Live rather than Horizons Live but I've no idea how representative that is (there's no reason it should be!)
As Darrack says, the game universe is huge; even ignoring exploration the bubble has about 20,000 inhabited systems, each of which has on average more than ten places a player might be present but not visible outside of that location. So even if there were 100x more people playing the game you'd still very often be the only one about.
If you want to play with other players your best option is probably to find an active player group that focuses on a type of gameplay you like and is awake in your timezone, and find people to meet up with that way.