Important aspect to scouting:

Open gates vs closed gates.

When your gates are open the scouts garrisoned in your city fight against the invading scouts to prevent a scouting report. Defending scouts appear to have a bonus in the combat so you want to have a 2 to 1 ratio in attacking scouts. If you send 200 scouts an the opponent has 100 with gates open, you will kill all their scouts, gain honor and get your scouting report. The opponent will not get a report on their end but may notice 0 scouts and a loss in honor. If you fail the scout battle, the opponent will receive a report of your failed scouting attempt since his scouts are there to report back.

When gates are closed, number of scouts sent doesn't matter, your scouting mission is instantly a success and you get info based off your informatics level. Sending 1 scout is an easy check to see if gates are open/closed as long as the city has some scouts in it.