Spies: How many do you need to pretty much guarantee reveal communications?

I usually get enemy messeges every day from using around 9 spies. When I use 6 I'd say its about 60% of the time or something like that.

intell-spies

they collect info province buildings, when they will finish what they are building,

the amount of resources player has, diplomatic status with other nations, what trades they have made ,

they will also find out where thier spies are,

intercept mails between players, and can revel enemies troops on the map.

So to intercept mail may not be guaranteed considering that amount of tasks they have a choice of.