If no one buys cards in a 2- or 3-player game, 6 cards get discarded each round (2 * 3 or 3 * 2), but only 4 cards in a 4-player game. This would lead to the maximum number of rounds. I suspect 4-player games are the least common, though.
4 Player is the only way I really want to play TTA. The pacts are more balanced, and the need snipe cards is typically a little higher too. Time to play the game is most certainly not a problem either.