Quote Originally Posted by Ibn-Khaldun
I'm not sure but I think it is combined value or something .. the more spies you have in your city the more likely you will get the enemy spies ..
It's pretty clearly a cumulative value, both for detecting spies and opening gates.

However, I'm fairly convinced (anecdotally at least, I don't have hard numbers) that both values have a hardcoded ceiling. As an example: I regularly deploy 2-5 spies into any city I'm about to besiege. I've noticed that I'm never able to get a greater chance that 85% to open city gates, regardless of how many spies I use or how high their aggregate experience is. I think it's reasonable to infer that this also applies to "counter-espionage".