There are 2 Lv. X's in each english Booster Boxes (which contains 36 pack, each pack has 10 cards in it, in eachc pack there are 2 rares)... but sometimes there is only 1 lv x per box, also it can go up to 3 Lv X per box, but rarely.
Japanese Booster Box contains 3 Lv. X's, and in some JPN...