if youre serious about using spies (a useful tactic against superior numbers or tech) you really need 2-3+ provinces devoted to spy making

the reason being that you lose a spy a turn in any province with watch tower/border forts

once you start hitting about 10-15 spies in a region (assuming ai moves a large stack there to try and ensure loyalty) then after about 10 turns you will start to see the rebellions

one region being assaulted like this can drain a faction of units in about 20-30 turns making it easy to assault (or you can just attack the new rebel provinces)

of course its a micro management nightmare constantly moving a couple of spies every turn into a region to counter their losses but sometimes if youre seriously outnumbered it may be youre only option

sending the spies into distant regions is a good way to reposition opposing forces if they want to retain the province so its useful to get the ai to spread its forces more or pin them in certain places

its also worth using spies in areas hard to control like scotland or portugal as these are nightmares to retain if assaulted by spies