Someone asked me this elsewhere, so I figured I'd give my reply here, too, as well as ask for what experiences others have had.

Here's what I said:

"Personally, I think 4 is the sweet spot. This breaks easily into 2 pairs if the group needs to split up. I have had up to 7 players at times, and this is too much, IMHO, since there is simply not enough time to give each something to do. With smaller groups like 2 or 3, you have more time per PK, but the amount of PK interaction is lower, which is a pity. Hence, in my mind, 4 has emerged as the sweet spot. But it does depend on the GM and the players, too.

That being said, my two current campaigns have 5 and 6 players, respectively... :P"

I have played in groups (non-Pendragon) which had players from 2 to 10 or so. The latter was clearly way too many. Most of the players spent the session chatting with one another or doing something else, while waiting for their turn / something that they could contribute to. On the other end, 2 players means that both of them must be present and ready to do heavy mental lifting. Which might be hard after a long week at work.

My 2c:
I would agree that 4 is the right number of players. I have been playing long standing campaigns that lasted for several years. That is why I like to have 5 players. This means that when 1 cannot attend the game still can continue with 4. With only 4 players and one cannot come it may mean you skip the session all together.
The best part in this is that KAP is also a good game when you only have 2 or 3 players.

On the other hand KAP is less interesting when the number increases above 5. KAP is a game for personal glory and so each PK needs air time to shine. With more players this cannot achieved well, even less than other systems imho.

I think it depends on the players as well as the GM. If they do not want the in-depth air-time, 6 players can be done. The more in-depth they want, the fewer the players should be involved because not everyone will get the chance to shine as Cornelius mentions.

I like 4 best. I have made 5/6 work, but people get shuffled out. With 2 or 3, you don't have enough resources and some group dynamics are lost

Solo is a different ballgame - then you can go all in on Entourage, etc

I'm working with two players. What I've found is that it gives me more latitude to highlight, use and expand on NPCs/entourage, and ensure that if "someone needs to die" they get the short end of the stick.

Of course, bad rolls will hit the PCs too; I'm referring to dramatic story pressure. If I had four players or more, then one of them would have to bite it, and at three there would be a temptation. The players are not really used to dynastic play, so I'm going to have to find ways of easing them into that over time.

--Khanwulf