Apparently the lookout on a Frigate could spot another's topmast at about 12 miles in clear weather, and a larger vessels such as a First Rate at about 14. That dependent to a certain extent on the angle of the viewed ship to the viewer, and the amount of swell. I interpolate from this that a First Rate lookout could spot another First Rate at a little over 14 miles, as both ships had higher masts. Obviously the lookout on a Sloop would be able to see a shorter distance.

Depending on circumstances, and the paranoia of the Deck Officer, would determine the number of lookouts in action at any time. It would seem usual to have two up. One at the Fore, and one at the Main.

Rob.