Nearly all cameras have several AF points and will select the best focus point automatically - usually the one corresponding to the object nearest the camera. Most cameras allow you to choose the focus point yourself if you want to.
Below is a simulation of an 11-point focussing system in a digital SLR. The green rectangles indicate where the focussing points are, and those highlighted in red are the ones the camera is using to focus on this particular subject.
So how many AF points do you need? The number of AF points is used as a selling point for the camera, the assumption being that the more there are, the better the camera's autofocus system.
This is true, up to a point, but having lots of AF points can also be confusing. Sometimes you need to override the camera's selected focus point, and this can take longer when the camera's got 9, 11 or (on some Nikon models) 51 AF points to choose from. Don't forget that however AF points the camera has, it can only focus on one thing.
Cameras with many AF points may offer predictive autofocus, where the camera can follow the subject's movement across a number of AF points and anticipate the correct point of focus based on this movement. This may prove useful for action photography, though many photographers still prefer the simplicity of 'pre-focussing' manually.
Some older, simpler cameras may have as few as three AF points. This is not the disadvantage it might sound in general photography, since they'll be spread in a line across the frame and can cope with off-centre subjects very well. It's much easier to override simple systems like these too.
Indeed, many photographers prefer to use their cameras' centre AF point for focussing, even when the camera has many more.
