Inside the event chest there are probabilities of you can find the Medal of Valour prize or not. If you get it, you will get the 100 reformed prisoners, but if you don't, your number of prisoners won't go up this way. If you got one on the chest, please take into account that: 
You can accumulate as many medals as you wish, but they must be spent during the ongoing event, or they will be lost.
You can find your medal counter by tapping on the red event widget. There you will be able to use your medals (tapping on USE button) and open new tactical chests (tapping on OBTAIN button).