One of the DP5 boxes comes with 2 of the 3 Pixie Lv.Xs and I think Magnezone Lv.X, and the other comes with 1 of the 3 Pixie lv.Xs, Porygon Lv.X, and something else.
Someone is selling the cards in the selling area of this forum, and it seems like they only got 2 Lv.X in their one box. So I think it is safe to assume with that information, with TRUK's comments, and with master of puppet's friend getting 4 Lv.Xs in 2 boxes, that the average Lv.Xs in a box will be 2.
That means that if you want one of each Lv.X, then you will need to buy 3 Boxes. That means, if you want a playable set (2) of each Lv.X, you will need to buy 6 boxes. Even after that, you aren't guaranteed to get 2 of each since you can get any 2 of the 6 in each box. So you could end up with 3-4 of a single Lv.X and none of another.
I don't like the distribution of this set as much as I like of DP4/5. DP4/5, you could just buy 4 boxes, and would be guaranteed to get 2 of each of the 6 Lv.Xs. Because DP6 isn't split into two sets, it makes it much harder to get 2 sets of all the Lv.Xs, even with the number of Lv.Xs in the set not increasing or decreasing.