So, well, I opened 12,000 basic boxes yesterday.

I screenshoted em, and here is the number of pieces, at any rarity.
I counted the Legendaries, Epics and Rares and there is a good chance that I misscounted by an unknown number.

1. Common: 9024 (75,2%)
2. Rare: 2562 (21,35%)
3. Epic: 355 (~2,95%)
4. Legendary: 59 (~0,49%)