For Cloud of Witnesses and Revelation of John, there is exactly one Ultra-rare per box. Therefore you would need to open 3 boxes to give yourself a chance of getting all 3 ultra-rares. However, my suggestion would be to start with 2 boxes (48 packs)--that will likely give you a complete set minus 1 ultra rare (or 2 if you happened to get the same one in both boxes) and possibly a couple of the rares, but certainly enough duplicate rares that you could trade for the rest. Obtaining the third ultra-rare will depend on which one you need. The Second Coming (CoW) and Three Woes (RoJ) are the big "chase" cards from those sets, but the other Ultra-rares are not that difficult to obtain.
