First. You have one village with scouts. 3k in the first months will do, then try to increase it little by little. I never had more than 8k, but you can go as far as you want. this is for real scouting, against big villages, heavily supported or against offensive players that cover themselves with scouts as they are too exposed.
Second. Defensive scouts. depending on the position of your villages, you should protect them with 50 -400 scouts, maybe more for very exposed villages. it is true that you can send some from your scouting village(s)but there are some problems that show of with the increasing number of villages. You forget to send in some places, you get them killed and you again forget to resend and so on. But if you go to recruiting area and you do not see the scouts, and decide to build scouts in every village, you always know that you have to have them. Another problem is that you begin to drain your scouting villages making them less effective. When you attack with scouts, your losses depend on the discrepancy between the 2 amounts. The more-the merrier. And if you care so much about farm space, with defense it does not matter, you build a little more in a scouting village if you want to even the numbers.
Third and very important.
Scouts used as offense. In the attacking villages if you put something between 50 and 200 you will get bonus information that has great price for you, even if you win the combat: troops in transit, resource levels, resources in warehouse and so on. If you lose and they survive and they do a lot of times, you at least know what is left there. So, for attacking villages, scouts are a must to build in that village.
For defensive villages. There are many times when you build nobles there. You than also have bonus info. Again they must be sent with fakes. Again bonus info.
So. the main reason for scouts is gathering intelligence - something more valuable than troops and sometimes skill. So use them at their best.
Note: Bear in mind that much of this is written with old units in mind and the old scouting rules. With new scouting rules and the inability(without paladin) to see outside units and the fact that it allows for general dodges to effectively combat scouts it severely lowers the effectiveness of scouts.