Are you using on-camera flashes that do TTL or a strobe system that can be controlled remotely? Yes, you need one PW device for each strobe you want to fire, unless you're plugging the PW into a pack. Basically there are 2 types of PocketWizard systems:
PlusII/PlusIII/PlusX/MultiMax - these are basic 'fire' commands to a strobe. You get to adjust each strobe independently of each other, but this can be a pain. The MultiMax can be setup in zones, and you can turn on and off each zone from the master. These are best as transceivers, as you can use 1 on camera, and 3 on strobes, or 2 different cameras each controlling their own strobes (if you set them to different channels).
FlexTT/MiniTT - these have transmitter only and transceiver models, and are designed for situations where you're controlling power manually or via eTTL. The times I've played with them, the MiniTT1 was more of a pain than it's worth, and it was just better to work with all TT5's. Now the key thing to remember is the AC3 accessory needs a TT1 or TT5 to sit in (this confused me until I played with it).
Depending on the strobes you have, there are additional modules for specific brands, so you may or may not need certain pieces. Then again, depending on what you're using for flashes, you can also set them do optical slaves, so as when one fires, it triggers the others to blink (not good when other flashes are going off).
