If you have seen samurai movies, or any kind of movies involving fighting, you might have at some point wondered why do the baddies attack the hero one at a time? Well, in the real world, no one takes turn in such fights. So why should we practice fighting one opponent at a time? At any given moment, one must be aware of his/her surroundings: the number of attackers and their positions. We must develop the ability to not only gauge the distance from each opponent (間合い), but also the relative distances between each of the attackers. This is not a simple skill to develop, so we start with two attackers. For simplicity, we often imagine these 2 opponents to be on two side with you sandwiched in the middle. You will decide who is closer or which is a more direct threat and strike him first, followed by a swift turn around to strike the one behind. This movement can be done with or without taking a step.
Once you have mastered the forward and rear strike movements, you can change the direction and achieve 360° steps. Combining different area of strikes (head, neck, arms, trunk and lower limbs) you are on your way to hitting multiple INANIMATE targets! Remember, if you were engaged in a real fight against multiple attackers, merely knowing how to strike won't keep you alive for too long. But this is somewhere to start with.
Congratulations! And welcome to the complex world of Iaido, where the more you learn, the more you realise you don't know.