"These resourceful units are vital to any commander. Scouts move behind enemy lines to provide intelligence, and are more effective in larger numbers."
Not to be confused with Scouting
Scouts are the first obtainable Cavalry unit. They are unique in the fact that they can only fight other Scouts, and are the only troop that can be used in a Scouting raid.
To enlist Scouts, only a Stables at level 1 must be built. In turn, this requires a Basilica at level 6. Scouts also cost 40 Iron and 120 Gold to enlist.
Although some players place Stables in a dedicated Infantry city to provide Scouts, it is wise to enlist Scouts in dedicated Cavalry cities. This demands only Barracks and Stables in a city to make enlistment speed as fast as possible.
As hinted in in the description, Scouts are most effective in large groups. A group of 50 or 100 Scouts would be fine for a 3000-5000 score city. For a "noob" city (1-1000 score) less may be needed: only 10-30 Scouts. Some players choose to bring a single Scout to a plunder raid to check if there are leftover resources after the plunder.