Somebody else helped out with this but I dont remember to whon I should give the credit, anyway, here is goes:
For a successful scounting mission you need to have the same or hogher level of informatics. For example: if you are scouting a L5 spot, you need to have researched informatics level 5 or higher. If the scouting fails with giving you numbers them scout again, and again untill you get what you need. Sometimes several scouting missions are needed.
I've been told that it doesnt matter how many scouts you send on the mission, people say that they have been successful with sending just 1 scout.
