I voted for 5.
"Manageable" may not be the right word. I could operate a game with 6+ players. "Quality" might be a better word. As in, "What is the maximum number of players before the quality of the individual experience starts to be affected?"
I've played in groups with as few as 3 players, and as many as 12 players, with the typical range being 4-6 players. The games with 10+ were truly unmanageable.
Above 5, I feel you start making trade-offs in terms of being engaged between turns, time in the spotlight, etc. The quality decline would vary greatly based on the playstyle, but I think 5 is a solid number for the majority of groups.