Each factory sealed case should get you:
12 different Rares (out of the 24 Rares in a set). Mistakes do occasionally happen but are relatively uncommon.
At least 1 of each of the 24 Uncommon minis in the set. You should also get doubles of 12 of the 24 Uncommon minis.
4 each of the 12 Common minis.
The way the boosters are distributed in cases now, unless there is a mistake (or it isn't a factory sealed case), the Common and Uncommon minis should be evenly distributed in each case. Therefore, if you got a second case, between the 2 cases you should have 2-4 of each Uncommon and 8 of each Common. If you have anything outside of this then either WotC stuffed up their case distribution or you didn't get a factory sealed case.
Rares, on the other hand, are a randomly distributed. If you got 2 cases then, on average, you could expect to get 18 of the 24 Rares in the set, Of course, having said that, it is possible (but unlikey) that you would be able to get all 24 Rares by buying 2 cases. There is also the (again unlikely) chance that you get doubles of 12 Rares.
I do not know if charlesatan is correct in saying that you can get a complete set by buying 2 sequential cases. I hadn't heard that before. I did remember reading on Maxminis that someone got triples of 12 Rares from 3 cases in either the Unhallowed or Night Below release. I think something must have gone wrong with the Rare distribution for that to have happened.
Olaf the Stout