Personally, I prefer a slightly larger group. Somewhere between 4-6 would be my ideal as it strikes the balance between lots of varied player motivations/styles and giving everyone enough time to roleplay.
The largest I’ve ever run is a 9 player table and that was a bit of a nightmare. The only reason it worked at all is because all of the players were good friends and they ended up splitting the party so I was effectively running 2 simultaneous adventures. My brain almost melted!
I don’t like to run anything with less than 3 players because I really enjoy the chaos that comes with fun PC conflict. If you have a tiny, intimate group and players decide to handle things in different ways, I’ve found that some players can feel more like it’s a personal affront, rather than just the game going in a different direction.
I’ve never run a 1-on-1 adventure and I’ve heard that it can be fun but, for me, there’s something a little awkward about it. I’m not sure exactly why and I know that lots of people like that but I think it’s just not my cup of tea.