i dont think only one would be enough...i saw a sick play where there was a carrier fleet with a observer group of like 5 or 6, the carrier fleet went ahead a little leaving the observers, the terrans comsat the area, flare all the observers, and the protoss player loses 8 carriers to cloaked wraiths.
even without medics, rest assured that the opponent will probably go for the observers first if they are using stealth.