One important consideration in any self-defense program is that of dealing with multiple attackers. It is common that an attacker will be among friends, or that a group of people will plan a surprise attack on an unsuspecting victim.
One strategy for dealing with multiple attackers is by leveraging psychology. The first thing is to assume that multiple attackers feel emboldened and safe because of their numerical advantage. It is possible to shatter that illusion of safety by handling the "ringleader" or the most intimidating of them in a swift and vicious manner. In doing so, the rest become demoralized; they individually know they are not as capable as the "best" of them, so they automatically know that a) they will lose against you, and b) you are not intimidated by their numbers. In this way, you are able to turn a many-vs-one scenario into a one-vs-one scenario. The rest will want to discontinue the altercation.
Perhaps the most effective strategy for dealing with multiple attackers is to utilize ways to make their numbers obsolete. One can maneuver and take advantage of a space, terrain, obstacles, etc. to great effect; the goal is to position yourself so that you are only fighting one person at a time, at all times. Sometimes this is not always possible, but one should strive to find ways to maneuver to more advantageous positioning.
In figure 1, blue is attacked by two red. In this scenario, it is two against one. Blue's strategy should be to maneuver to a position similar to figure 2, where one attacker is positioned in the way of the other, so it's one against one. Another way to facilitate this is when an altercation takes place in a confined area, such as a hallway. A great example of this effect is in the Korean movie Old Boy, where a hallway provides a way for him to fight multiple attackers as if they were one-on-one (see video below).
In figure 3, blue is surrounded by three attackers, and there is a wall behind. In this scenario, it may not be possible to force a one-on-one, but blue can utilize the wall to prevent an attacker from moving behind. In addition, blue can attempt to maneuver so that it is momentarily one-on-one, and provide himself with an exit or to place the attacker against the wall, which may be advantageous if the attacker is uncomfortable in such a situation.
In this scene, it start out rough, but he eventually moves to the end of the hallway so he can fight them one on one.