1. Balance. The scope and magnitude of your adventures should be proportionate to the size of the group. Don't make the mistake of coming up with an epic-sized adventure before you find out that only three players will be attending. Sort out exactly how many will be playing before you lay the ground work for the stories. Small groups can be overwhelmed by the scope of the events unfolding around them. Likewise, large groups can be bored to tears if the scope is too small. Also, especially for smaller groups of 2 to 4, focus your storytelling on the character and how the develop over the long term rather than a string of events climaxing in a grand battle. On the other end of the spectrum: players, don't worry about whether or not your party has a "healer" or "tank" until after the players have been established. The idea is to not limit your choices. But truly, a party can be comprised of any mixture of archetypes and be successful so long as your DM/GM/storyteller/referee/whatever can manage who is doing what in the group. Fights are always exciting when driving a story forward.
2. Groups can be rather large, but we say six to eight people is pretty darn large for a group. Any more than that, and you risk losing players' agendas in the mix. Nobody likes putting effort into creating a fun backstory and then it gets ignored. BOO FOR THAT! If whomever is running your game is comfortable with ten or more, then who are we to argue. But just because you know 20 gamers, doesn't mean you have to have 20 people in your game. Factor in you preferred mode of storytelling when deciding who to invite. Remember players: you are not obligated to participate in a game if you feel uncomfortable with the number of players. Exercise your freedom if you don't want to be in a large group. Gaming is not a chore, nor is it an iron-clad commitment. It is just a game.
3. When running for four or more people, keep in mind that more doesn't necessarily mean merrier. Whatever happens, keep the action moving! Experienced players should be leading by example and know what spells they have at the ready and what sorts of abilities they have at their disposal so that when prompted, they can resolve their attacks or actions quickly and efficiently. Experienced players who are indecisive or constantly look up stats or abilities in books, or in other ways stall the action should be passed over and allowed to resolve their actions at a later time. Adventure waits for no adventurer! We're pretty sure that Sauron didn't wait for Frodo and company to look up what they wanted to multiclass into when they gained enough XP.
Bonus XP: For even the smallest groups, splitting thing up to keep the plot moving is perfectly acceptable. Safety isn't always in numbers. Proper teamwork can overcome almost any problem. This also promotes inter-party relations and builds a higher sense of cohesion. Even the most epic characters in literature aren't powerful because of how many attacks they have. They become epic because of what they learn from the people they know. In the end, it doesn't matter how many players are participating, just that they are participating and having fun; the ultimate goal of any good game.