For my 2222nd post (!) I’m tempted to ramble about magic numbers again, but I’ll spare you...
Instead... two-player games.
Batman and Robin, Han and Chewbacca, Fafhrd and the Gray Mouser, most romcoms, there are a variety of two-character groups out there.
Only having two players means that one missing a session results in cancellation, so it’s not so common in gaming. This is a shame, because it works better for some story types than having a gang of people around.
Speaking personally, back in the day I was one of three really keen geeks in my high school, so we had a few games like this as well as some larger ones. More recently I ran a season of my Doctor Who game with the two keenest players bouncing ideas and dialogue off each other as their characters dashed across all of time and space, much closer to the classic Doctor and companion style, sadly curtailed because one moved away.
Other than that, I’ve had a few two-player sessions of larger games - it’s the least I can GM for without warning. Of course that depends on the players and PCs - tonight I cancelled a session that would have had an R2 unit teaming up with a technophobic martial artist.
If I had two keen people, I’d love to try something like Never Let Go...