if you have more of the same kind of the rebellions they will stack (but i don't know the formula). For example, it's easier to control 10 province with 5% each of unrest of ten different kind of rebels than 5 province with 5% each of unrest with the same kind of unrest (and harsh treatment will be more expansive).
To manage rebels there are some ways:
1) harsh treatment
2) position army on province with unrest will lower the unrest (depending on the size of army)
3) rise autonomy in a province will be lower the unrest of 10% of that province
4) if rebels spawn, for a short period the unrest will be set to 0 in the provinces where rebels spawn (i'm not sure how long is the "period" but i think some years)
5) core, convert and change culture will lower the unrest (but during the coring, converting and changing culture the unrest will be higher than usual)
6) high stability and legittimacy will lower the total unrest
7) humanistic idea give -2% total unrest (if i remember correctly there is also another idea group that will lower unrest, don't remember what, just check them)
8) Overxtension and war exhaustion will rise the total unrest (and also religious unity if it's lower than 100%)
9) nationalism modifier will rise the unrest (normally it sets at +15% and decade -0,5 every years so after 30 years, it should go away if no rebeles occupy that country and humanistic idea will lower the duration of nationalism)
anyway rebeles who will spawn by events will be not considered for the unrest, so if some kind of rebels are likely to spawn and then by event in thos province rebels will spawn, the unrest will not go away