Your ability to scan is limited by the size of your scanner on your ship. (Ok, keep it clean)
You can have a trillion fighters and torpedo's on a planet. There is a formula for how production of fighters and torps are governed, but as far as I know there is no limit to how many a planet can have.
NOW, fighters deployed as sector defense have to have ENERGY from the planets, or they will reduce in numbers over time.
Scanning a planet is to determine whether or not your ship is capible of taking on that planet. For the most part, you will want a ship that has a huge beam, armor, shield and power setting. Pack in as many fighters and torps as you can, (30 + hull size is good) and you can take on just about any planet. That is unless you do what I did, and try to take one of the WebMasters. Big mistake.
I spent 5k turns setting up my ship, and attacked one of his planets in sector 499, and was defeated soundly. (Restart)