TItanfall has a confirmed 12-player headcount, but that doesn't reflect any limitations on its part. In reality, every map (or match) in Titanfall can support around 50 characters, including all the AI running around alongside players.
Each of Titanfall's maps can support up to 12 AI characters on each team, and each player can potentially have a Titan folliowing them around. That's a possible 48 characters on the map at once.
So why did Respawn decide to go with the six-man teams instead? Apparently, that's for the player's benefit. Based on feedback from play testers, a crowded map just isn't as much fun. With too many people and too many things happening at once, the experience becomes just a bit off-putting.
Lead designer Justin Hendry explained the problem:
"The higher the player count, the more uncomfortable the game gets," he said. "Unlike in most games where you can sit there and guard the two ways in, in Titanfall the guy can come in through the window right behind you, he can come from the window to your left, he can come from straight ahead, he can come in from the stairway and he can come in from the doorway, or whatever. Essentially there are five directions you can get killed from and the higher that player count, the more likely you are to get killed from behind and the more difficult it is to kind of manage your surroundings."
The reason such a high player count wouldn't work for Titanfall is its high mobility gameplay. Players aren't just running across flat terrain, but performing wall runs. This singular feature can easily overwhelm our poor brains, as players then have to pay that much more attention to everything around them, from enemies above to the Titan ahead of you. Think of it as an overdose of action.
During development, Respawn considered a number of different team sizes, even two-player teams. Eventually, they went with a six-versus-six setup. Despite some negative responses, the developers are confident that players will eventually come around.
"When people start playing Titanfall like Titanfall, the player count becomes a non-issue," Hendry added.