Just use the "Alian Assault Traders" forum for general question.
Well, planets can hold 10 enemy spies from ONE player. But you could have 30 enemy spies from 10 different players on a planet doing nasty things.
And there isn't a 100% guaranteed way to remove spies. The level 3 cleaning will do the best job at finding them but that doesn't mean it will clean out ALL of them.
And how spies are found is basically correct. When a spy is placed on a planet it takes the cloak value of the ship that placed it there. That cloak value will never change. When you try to clean your ship of spies it compares the level of the cloak on the slie to the sensors on the ship/planet. The higher your sensors are than the spies cloak the more likely you will find them. But the spy having a higher cloak than the sensors doesn't mean the spy is 100% safe. It can still be cleaned though it becomes harder the higher the spy cloak is to the sensors.
Now there is a plus here... If you own a planet and have spies on it they will HELP your sensors in finding cloaked enemy spies. The more spies you have the more they help. So having 10 spies really helps in finding that cloaked spy.
The spies you are carrying on your ship will do nothing to help find enemy spies on your ship.
I hope that helps a little.