Thanks, hot take incoming!
I think three is my favourite. Small enough to have quite some time in the spotlight, big enough to have a chance to relax, sit back, and see what the others do (both as a player and as a GM), or to brainstorm for ideas.
Two and four are still good, even though with two gets pretty demanding for everyone, and with four quiet people need some extra help. Five is too much of a stretch and something I’d do only very occasionally and only with some certain games. Above that is a no go for me.
For two players total I’d rather bring out a game specifically designed for that, or something GM-less.
In a sense I have a similar range, as I could in principle run a game or campaign for two people, and at the other end I would never choose to run anything with six or more regular players (I have run sessions with five regular players and a guest star playing a character I had created). However, five seems to be my most frequent number of players to invite into a campaign. I’ve run a number of successful campaigns with four; I’ve run successful sessions with three, when some players were unavailable, but at least when I was in San Diego and had a big circle of interested players, I would never have felt right about running anything for three or two players, and my GMing style is really optimized for bigger groups.
In the campaign I’m now running via Zoom for four players in San Diego county and one in Nevada (Reno, I think), there’s a good deal of both world exploration and character interaction. The highlight of the latest session, for example, was one player having his character consult with his wife and decide that they would inquire into whether two of his shipmates (the cook and the ship’s girl) would marry the two of them (I rolled for reactions and found that the ship’s girl liked the idea but wasn’t free to do so until the current voyage was over, but the cook was at best reluctant).