Originally Posted by JLyon
Actually you could get away with 6-12 more officials, by having them calling the far sideline only, the chair can usually see the rest of the lines pretty good.
You'd still have spots that don't get a good angle to look at. In volleyball, they stand right on the corner with a perfect line of sight down a baseline and sideline. Due to the net in tennis and the service line, you need 5 per court to have someone at the fence calling a line.
Even then, you'd have interference issues with finding places for the line judges to sit.