Ahhh, after many, many years of GMing, I arrived at a compromise for me and my players. I created a campaign which required them to be together and gave them direction (they are trouble-shooters for the king of a particular kingdom). They pose as a warband to hide their true identity and missions. The Crown sometimes gives them missions and sometimes the pcs must take missions totally unrelated to the Crown to keep their cover stories. This allows the pcs the amount of freedom they want and allows me the control of events I need to fabricate a good plot and story. I make it known to the pcs from the beginning that they are part of a elite band of dedicated individuals who get special freedoms from the Crown. If the pcs are not part of this elite force, then they are not pcs in the campaign. That simple. I've defined the environment and the players can choose to play in it or not. My group has ranged from 5 players to 12 (currently at a healthy 6 with a 7th being on hiatus).
I don't railroad them, I assign them tasks because, it's their character's jobs. By making the campaign as I did, I provided focus from the start and even though the campaign is lousy with side plots, sub plots, and underhanded plots, there is still the focus of working for the king.
All this is to say, give the campaign a solid, interesting focus and you won't have to work too hard keeping up with the players.
I don't railroad them, I assign them tasks because, it's their character's jobs. By making the campaign as I did, I provided focus from the start and even though the campaign is lousy with side plots, sub plots, and underhanded plots, there is still the focus of working for the king.
All this is to say, give the campaign a solid, interesting focus and you won't have to work too hard keeping up with the players.