Actually, I was thinking you should start with 3 pairs...and then add a 4th pair if necessary!
That is, while I don't have the Dispatch Board I would have thunk it was designed to operate well with 14V AC which I figure is the most common Accessory AC voltage in O-gauge. 3 pairs drops the 18V AC to about 15V AC...and 4 pairs drops it to about 14V AC. I guess it's a question of how much exercise the man needs!