Flexibility of number of players in Fluxx
In my group, Fluxx tends to end by accident, regardless of number of players.  Actively trying to win pretty much ensures that you won't.  Having lots of players (I think my largest was 10) doesn't seem to change that.  I've had large games end in 5 minutes and I've played in games where we got bored and quit.  I see what you mean by your question though.  Intuitively it seems like a large group would make it harder to win.  In practice, I think there are so many ways to win that the number of players doesn't really change the game.

The only group size where Fluxx isn't as much fun is 2.