you can set it as high as you want. i usually use something like 9999999999999999999. in my old roman game with that setting i got to about 230BC before I experienced my first random rebel spawn.

i'm not entirely sure, but i think that the stat basically means the chance that a rebel army will spawn in each region. for example if its set to 100, there will be a 1 in 100 chance that any region will experience a rebel spawn. the amount of unrest in the province also increases the chance of getting spawns. however, even if you run low taxes in every city you still get them pretty often.

i recommend keeping small armies in each major area when using a very high setting to make up for the money you save not having to fight them, if you decide to do that.