What's an instance? If you're talking human and not NPC controlled ships then:
The most open symbols I've ever seen at once is 2. Occasionally 1. Usually none.
Spawned 1st ship in Federation space. Been there ever since and don't think I've ever been much further than 60Ly or so from SOL. Does that make a difference?
It varies. We've all got rather spread out.
I've become 'friend' with about 8 Cmdrs, but none of us is anywhere near each other in galaxy and yet we were in the same systems once.
I've said a 'hello' to the odd one I see, but short of interdicting them, I rarely get much answer. They are usually just passing through.
I'm based around the Vequess system, but had been out in the sticks.
Try going to the Lave systems, I believe a lot of players head that way. It'll be mostly people who remember the original game.

I would have though the places you find most populated might be found by following the Galnet news feeds.
There's 400 billion stars out there, even within a 60LY radius there must be hundreds if not thousands. the chances of seeing other's without planning it are quite remote I would think.
If you see people, say hi, some will be happy to chat, some not.
