If you are indoor, you can usually get by with one trigger and one receiver if the rest of your strobes have an optical slave (most monolights have that built in - if you are using speedlights for stobes, most require you to get an external optical slave (I think some of the Nikons may have them built in). An optical slave senses another flash going off, and triggers the flash it is connected to.
If the strobe doesn't have an optical slave, you need a receiver for each light.
When shooting outdoors, optical slaves are of limited effectiveness, and you usually need a receiver per light.
