The traitor count is fixed by how many sit down — roughly one in three. Quest sizes are fixed too. At seven players or more the fourth quest needs two Fails to sink it; above ten, so does the third.
Three quests held and the realm stands — unless the Assassin then names Merlin correctly, which takes the game for evil at the last breath. Three quests lost, or five parties turned away in a row, and the realm falls.
Merlin, Percival, Guinevere, Tristan and Isolde, the loyal Lancelot and the loyal servants stand for the realm. The Assassin, Morgana, Mordred, Oberon, the fallen Lancelot and the minions stand against it. Every world renames them; the abilities never change.
Avalon is printed for five to ten. Decevia goes to eighteen, with the team split and the quest sizes extrapolated from the printed table's own arithmetic rather than invented.