Scout plans give both global intel about the country but also local intel such as how the divisions are positioned in the air zone they operate in. The more Aircrafts you have the more detailed the information seems to become, even if your global intel about the country change. How many Aircrafts you need per zone is not clear, but the more seems to give somewhat better information, like 2k did somewhat better than 1k but the difference was quite small. I don't think you need that many to get quite good information about the enemy division placement, like maybe a few hundreds per zone would be enough.
I tested as Germany against Poland, no matter how many Aircrafts I putted in the western Poland airzone, no information was provided about divisions in Eastern Poland, but as soon as I added Aircrafts to that zone I started to see the divisions in Eastern Poland.