A two shot is a type of shot in which the frame encompasses a view of two people (the subjects). The subjects do not have to be next to each other, and there are many common two-shots which have one subject in the foreground and the other subject in the background. It is very useful if the film is about two people.
The shots are also used to show the emotional reactions between the subjects. For instance, in the movie Stand By Me, this shot is used multiple times to show these emotions.
An "American two shot" shows the two heads facing each other in profile to the camera.
A "two shot west" is a melodramatic device used primarily in American soap operas. In a two shot west, one character will turn 180° and face away from the other character while they continue to talk, which enables both characters to appear together in a single shot facing the audience.
Similarly, a three shot has three people in the composition of the frame. In these shots the characters are given more importance; this type of image can also be seen in print advertising.
It was used heavily in The Good, the Bad, and the Ugly as facial expressions were an important aspect.
Logically, a one shot would imply a single person visible in the frame. However, this phrase also has another meaning as in a single take (continuous footage, with no cuts).