I made an Excel sheet of the miniatures list from the Icons of the Realms line. I marked the ones that are also in Attack Wing so if you want to wait a couple of months you can get those another way. From the releases images it looks like they will have different paint jobs in each set. All info is from the CoolStuffInc preorder page but it matches what I've seen elsewhere.
There are 13 common, 19 uncommon, and 12 rare minis (not including the starter set and the Gargantuan Brass Dragon). This makes me think there may not be a rare in every booster.