Many a DM is put off by large groups of players. Sure it seems like a daunting task keeping track of the actions and comments of a dozen or more people but it need not be seen as an overwhelming task. For me large groups of players are when it's time to shine.
A place to play:
To me the greatest obstacle of all with large groups is a place to play. With a large group the only thing I've ever seen work is a regularly scheduled place the DM has easy access to.
The physical reality of fitting a bunch of players together can be difficult. There are two sensible ways to deal with this, a large table and seating in the round.
Not everyone has enough space for a large table or space to keep one indecently. A couple portable tables solves the big enough table question. I've got 2 camp tables to create table space when needed, there still has to be enough room so this isn't a solution for everyone.
Seating in the round puts players all over the room with the DM to one side. All players should have a clear view to the DM and should have some playing surface nearby. A folding table, a coffee table and such. For more theatrical DMs this also give them room to put on a show.
Chit-chat and breaks:
Chit-chat between many players can be deafening as can players leaving the play area at random intervals.
All the players have to be a little considerate of others and be sure to leave the DM with speaking time. A DM shouldn't be a tyrant in regards to chatting but should discourage it. One method I use is :when players distracted and chatting so are their characters. A little room has to be given to allow players to discuss tactics and shout advice but not too much. Use of digital devices should be curtailed as well, no frequent texting, web-surfing or talking on a phone in the play area is polite and avoids disrupting the game.
Timed Breaks are a great way to reduce chit-chat and people getting up from the table. Every hour-two hours have a break planned at a minute per player. This gives times for refilling snacks, bathroom runs and cigarette breaks. If that seem too frequent plan a larger break in the middle of the play session.For a time when we were younger and played an every other week sunday game we would break in the middle of the game for half an hour to an hour to play frisbee or football or bet each other with buffer weapons. Such breaks let people deal with necessities and gives the DM time slots to adjust notes.
Another tactic that works great to reduce chit chat is the pre-game gathering. Plan to have a meal or some other activity before the game-play starts. For years on my sunday games it was Robin Hood Brunch where we'd get together have a shared brunch while we watched Robin of Sherwood, it was a great mood setter and gave time for a lot of chit-chat to be completed and everyone ate before the game.
Pacing and Style:
The DM needs to make sure the play style used assists in the play of the game and doesn't get in the way.
Keep the players reacting to the DM not the DM reacting to the players. With a large group there's going to be a lot of players pulling the game in at least as many directions. The DM need not railroad but with a lot of players it's helpful to wave a red flag in front of them now and again and focus the players actions.
Let the party split up. This seems contradictory to reducing chaos but it's not. Allowing players to split up while acknowledging only so much time will be spent in a segment of play with split off groups will keep some players from stealing the limelight and keep the action going. It gives players time to neaten up notes and take unplanned breaks while other players are involved in activities they aren't. Be mellow on dealing with players knowing things the PCs wouldn't. In my own games I've used surprise engagement rules where groups of PCs have engaged each other briefly when bumping into each other in the same dungeon.
Use cliff-hangers. Don't' always stop the action at a point of resolution. Let moments of great anticipation be when you shift between groups of separated characters or call a break a couple minutes early. "Oh no the floor upend under Pherd…" is much more dramatic and keeps players attention longer then "Oh no the floor opens under Pherd, he falls 20 feet for 6 points of damage".
Have uninvolved players hoping the DM. Want the players to have a tough fight with a group of monsters? Imagine the carnage with guilt free attacks from another player. I'll have players take the roll of faceless-goons or wild beasts now and gain (if they wish) to keep them busy at the table. It's a fun change of pace for the players and deflects the everyone vs the DM dynamic.
Let the players decide what the they do. While I recommend keeping the players in a position they react to the DM it's essential that in large groups the players feel they are in control because each will be having less table-time. The DM should wave the red flags but shouldn't nudge players decisions otherwise, let them sink or swim by their own decisions not ones you make for them.
The DM must keep notes on player statistics, locations and time events occur. It's always the rule it's even more important with many players wandering about. All you need is a notebook or pile of notecards, along with the discipline to record notes and the problem is solved.
More then one character:
This seems totally insane when dealing with 12 or more players why would any DM want to deal with more characters? For one thing it keeps players involved and remember we are allowing the layers to split up, a player will end up with characters spread around not all marching together. It also keeps players from being drama-queens when a character is slain,losing one of 2 or 3 characters doesn't disrupt the game. I know form experience a player will either foster a number of fun characters or end up focusing on one central character and keep a few active pawns running about in the background to pick up the slack or have something to do when their main character is involved off-stage.
Keep the players informed. My old character sheets had play notes/aids on them they weren't just for keeping stats. Make a few posters with notes on them for players to reference in play, these need not be fancy, all yuo need is some notes written large enough to read.
Make a newsletter. a small newsletter is a great way to keep players informed without taking up table time. This can be done with a blog but folks have to remember to check a blog, shove a piece of paper in their hand and they are more likely to read it. I did mine as a 1 or 2 sheet faux-newspaper for a while and it was a little silly it was fun and a great way to communicate trivia about the campaign without cramming it into play at the table. A good way to make sure peopel read the newsletter is to distribute one or two less then the number of players at the table this keeps people interested in the newsletter and those that don't read them much may take a tiny bit more time reading it before they pass it on to another player.
Make sure the end of each session is a planned debriefing period. Players and DM update each other on their notes and the DM issues any EXP earned. End of game time shouldn't be when everyone runs for the door. It also gives the DM and host time to deal with slow to leave players. Some players should stay to help the host clean to ensure good will and make sure the place is accessible in the future.
Lest readers think I'm talking from outside my experience let me assure you i ran a 7-8 year long campaign that followed these loose guidelines. We had about a dozen regular players, many had 2 or 3 characters. At one point in the campaign the PCs were scattered all over the place in three time zones, a couple planes and a few locations in the main campaign area and it worked well. When one player couldn't make it the game didn't collapse because Rothgar the Red wasn't there with the campaign advancing MacGuffin.
Keep the players busy, don't let there be time to be bored, keep them reacting, and keep them in the game.