In general terms, it is advisable to have three scouts, and I prefer to have them in three different regions. If you want to train only certain skills, you may ask the scout to look only for the kind of players you need, but it doesn't seem to have a big influence in the players who he shows you. However, given that the manual says that when looking for an specific kind of player, it is more likely to find good players, it is always advisable to request a specific type of player. It also increases the risk of not finding anyone, but don't worry.