How many dimmer switches do I need?
Conventional switches

When you have two switches controlling one circuit, they are both 3-way switches They are called that because they have three active contacts, one common and two travelers. They also have a ground and may have a neutral for extra features.  

When you add any more switches beyond two, they each must be 4-way switches. These have two hot contacts in and two out and sit between the two three way switches (electrically in between, regardless of where they are physically). These also should have a ground contact and may have a neutral.

When you add a dimmer, it must replace one of the 3-ways, not the 4-ways. You also need a dimmer that is 3-way compatible (not all are).   Also, if you are using CFL or LED lights, make sure the dimmer is compatible (it should say so on the box).

Only one dimmer can be used. The other switches just turn the circuit on and off, at the level the dimmer has set.

Multiple dimmer systems

As @DMoore has said, there are newer electronic switch systems that allow dimming from multiple locations (as well as on and off). One switch is a master and the others are slaves. In effect, the master controls the circuit and the slaves send a signal to the master to do that remotely.

This system will give you more flexibility than a conventional switch system, but it is significantly more expensive. If you do not  need this flexibility, adding one 3-way dimmer is cheaper and easier.