One set is good for one off camera flash. Is that correct?
Not quite. You can have one ETTL flash on the transmitter's hotshoe, and another on the receiver hotshoe. Then a third (ETTL flash, other hotshoe flash or studio strobe) can be connected to the receiver by a PC-sync cord, but as a simple-trigger flash. They can be 1st, 2nd curtain or pre-shutter (HSS continuous light, or Supersync flash burn).