I don't often play medieval fantasy these days and to some degree that has it's benefits when it comes to scheduling and number of players. In some genres and situations, not all the players are needed ever adventure. If a group of PCs are travelling together and camp out, then next week two players can't show, their PCs are still at the campsite. This forces the GM to decide what to do with them.
My Mutants & Masterminds superhero game was set up like Justic League Unlimited. The 3 or 4 regular players were the 'Greatest Heroes on Earth'. Other players would come in and out as their schedules allowed and would essentially be 'guest stars'. They were assigned to mission when they showed and were somewhere off base when they didn't. Probably fighting crime in their hometown or something.
I ran several Star Trek ganes with a similar idea. I had a crew of 6 who were allows there and played the Captain, Engineer, Science Officer, Security Chief, Helmsman and Doctor.
One friend of my could only show up when we ran games on Sundays, which was fairly often, so he played Navigator. There were two guys who could rarely show, one a GM of his own Champoins game and one was a friend of ours whose job had terrible hours. The first guy 'guest starred' as an Ambassador who would come along on important first contact missions. The latter wanted to be a Shuttlecraft Pilot. It was hilarious because if he showed up you knew we couldn't beam down for some reason and had to take the shuttle, lol.
Player 1:" Is everyone here? Can you start?"
GM:"Not yet. I'm waiting for...*Sound of the door buzzer*. Ah!"
Player 2:"Hi guys! I can play today."
Player 1: "Hmm...Jeff's here. There seems to be an ion storm disturbance in the atmosphere. We're going to need to use the Shuttlecraft. Funny that seems to happen only when Jeff comes over..."
Also, not every actor was in every episode of each series. Once is another ST campaign, we planned a big adventure and then a snowstorm hit and only 3 players out of 8 showed up. I created an adventure on the fly in which the three players were attending an annual Robotics Conference on a planet while the rest of the ship and crew were doing a routine survey mission. Turned out to be one of my best side adventures ever.
AD