It really depends on the kind of game you want to run. Games that focus on character growth and individual storylines cannot survive more than a handful of players (3 is best, 5 is the max). Games that focus on adventure, exploration, combat, etc. work fairly well with larger groups, especially if you have irregular attendance. My current 5E game is running the Temple of Elemental Evil. I have 8 players, plus myself. We seldom have more than 6 players at a time, but even when we've had 7 (I don't think I've had a single game with all players at once), it worked fine.
Some tips:
* Have a Party Leader selected at the start of each session. They are responsible for telling you the group's action ("we go down the left hallway"). This is sometimes called the party spokesman or "maker of irrelevant decisions."
* When the group needs to make individual decisions (such as exploring a room, actions in town, etc.) make sure you go around the table so that everyone gets a chance to make a decision. Alternate which way you go around the table to keep the end players from always being on the spot.
* During Combat, make sure everyone is paying attention and planning out their next move, so that they can act immediately. This is the number one thing that will slow down combat. Turns themselves usually only take a few seconds; it's the decision that usually takes time. You need to do the same with the bad guys (which can be very hard, since you're also keeping track of everything else).
* Make sure that players have character sheets that give them all the information they need to play their character. The number two thing that slows combat down is a player needing to look up how something works. It can also slow down exploration and sometimes social interaction too, so this is really important.
* Have players roll damage at the same time they roll attacks; it seems silly, but it really does help.
Some tips:
* Have a Party Leader selected at the start of each session. They are responsible for telling you the group's action ("we go down the left hallway"). This is sometimes called the party spokesman or "maker of irrelevant decisions."
* When the group needs to make individual decisions (such as exploring a room, actions in town, etc.) make sure you go around the table so that everyone gets a chance to make a decision. Alternate which way you go around the table to keep the end players from always being on the spot.
* During Combat, make sure everyone is paying attention and planning out their next move, so that they can act immediately. This is the number one thing that will slow down combat. Turns themselves usually only take a few seconds; it's the decision that usually takes time. You need to do the same with the bad guys (which can be very hard, since you're also keeping track of everything else).
* Make sure that players have character sheets that give them all the information they need to play their character. The number two thing that slows combat down is a player needing to look up how something works. It can also slow down exploration and sometimes social interaction too, so this is really important.
* Have players roll damage at the same time they roll attacks; it seems silly, but it really does help.