I never use spies, they never seem to do anything. I once put 40 spies in 1 province, most of them valour 3 and it never went into rebellion. They very rarely manage to reveal a general's vices or open a castle's gates, because they are either killed by BFs or just fail over and over again and when they do they usually reveal pride and create jedi princes, or even worse jedi kataphraktoi and boyars.

The only usage for spies I can see is for your own personal pleasure, annoying your generals and for defense. They seem like a more functional replacement for border forts so you don't have to spend a few turns guarding a province after you've conquerred it whilst you build your BF and also once a spy captures loads of assassins you can send it out to attempt to do one of the useless things I mentionned.

How many spies equals the agent capturing power of a BF?

Where do you get all this information about the possibility of success?