The answer by Skiptron pretty much hits all the high points.
I'd like to add a couple of things to it though.
IRL Time Planning
Knowing that a break is coming will help prevent constant interruptions by players leaving the gaming area. It also gives the GM a chance to take a deep breath, gather their thoughts, and figure out what direction they are going to head next.
Make sure you plan on taking a short break every two hours or so. A good 10-15 minutes is enough time to make a critical phone call, use the bathroom, grab a snack, or just stretch. Try to pause on a cliffhanger if possible but don't stress over it.
Plan a longer break for food periodically. Depending on the venue it is a good idea to have the meals themselves planned or have a list of nearby options you can give the players.
Pros: You can tell part of your story int he character backgrounds. All the characters will fit the world you are running so you will not suffer a massively out of place character concept (I'm looking at you dual wielding drow rangers in sunlight). You can choose to not give the wizard the one spell that will completely bypass a cool encounter you have planned
Cons: That is a lot of work you have to do up front, especially for 20 players. You need to make more characters than players if you don't want someone to feel like they got "stuck" with the crappy one.
Player Built Characters
Pros: Get you better buy-in from the players themselves. Takes a lot of front-load work off the GM.
Cons: The under-powered, overpowered, and uninteresting character possibilities are endless.
If your willing to work closely with the players either one of these can work since you can mitigate the worst of the cons. It does require alot of player communication and time. To help deal with the communication issue during creation I would recommend a google doc for each character. Share it with all the GM's and the associated player.
Ignore Below - Left for reference, not needed by for answer after system tag was added
I divide most gaming systems into one of three major categories. Generally speaking the less rules the easier it is to run for a large group of people. I am sure there are tons of examples I don't know about or forgot to list here, please forgive me in advance.
These games have alot of specific rules and rely heavily on tactical combat. Tends to require Minis, Maps, Chart Look-ups, Lots of Dice, and/or Lots of Roll Modifiers. Gear tends to be a major factor in these types of games and can get very nit picky in the differences. Character sheets tend to be complicated and require 2+ pages to fix all the information in a well laid out manner.
Examples include DnD 3.5e, DnD 4e, Pathfinder, Iron Kingdoms RPG, Almost any 70's or 80's game (like BattleTech)
These games tend to be able to run mapless. Modifiers are generic in some way (Boost Die in EotE or Aspect in FATE) so they are easier to track. Character sheets tend to be less complicated and can fit on 1-2 pages easily. There is usually some mechanic for the players to drive the story which helps keep the characters involved.
Examples include DnD 5e, FATE, Edge of the Empire, Savage Worlds
Rule of Cool
These games are all about the narrative and the rules are just kinda tacked on to give a concrete way of resolving differences. Props are usually just that, props, and not required for game play. Character sheets are generally 1 page affairs and sometimes are only kept as a reference outside of where game play is taking place.
Examples include Storyteller System (World of Darkness) and the Dread RPG.