How many Transceiver's do you need?

[wideangle]

Would something like a PocketWizard Plus III Transceiver be able to be mounted to a camera hot shoe and trigger multiple external flashes? That is, do you only need 1 transceiver, or do you need more than one radio trigger in order to get other flashes to trigger?

[I @ M]

The answer is in the wording = transceiver, that is one unit is both a transmitter and a receiver therefore you need two units. One to transmit when the camera shutter button is depressed and one on an external flash to receive the signal and activate the flash. The ability to activate multiple flashes is determined by whether you wish to have a unit on each or for the flashes to be linked by a cable or for them to be activated visually from the first flash with the receiver attached. That will depend on whether your second and subsequent flashes have inbuilt or added slave sensors.

[Wobbles]

Would help if you were a bit more specific about what camera and flash brand/model combination you wanted to use...
Otherwise just buy a bunch of these http://www.hypop.com.au/yongnuo-rf603-c1-flash-trigger-canon-eos-60d-550d-450d-400d-350d-1000d.html and you can fire off as many flashes as you want relatively cheaply!

[wideangle]

I would like to be able to set of at least two external flashes setup on umbrellas. I would be using a Canon 7D, Canon 580EX, and a third party flash like a Young Nuo 560.

[chappo1]

As wobbles said, you will need a transmitter (or a transceiver) on the camera and a receiver (or transceiver ) on each flash. Or you could go with one receiver and an optical slave on the second flash....john

[raysul]

Talking as a beginner here, I have a Yong Nou 560 II, which only has M,S1 and S2 the same as your YN 560. Placing a Transceiver on your 7D and a second transceiver 580EX, and place YN560 mode to S1. So when your 580EX triggers so does your YN560

[chappo1]

Up until the A99 I have triggered off camera flash from the pop up flash on the camera but the A99 does not have a built in flash so I bought a pair of Pixel Opas recommended by a friend and they arrived Friday. Broadly similar quality range to the yongnuos mentioned but gives you something else to research.
Other than determine that yes they work, I have not explored them yet...john