Keys dropped: 10
Epic keys: 1
About 2000 battles; estimated from trophy count. Probably a bit more, with gnomes ~1/15 battles.
Based on the numbers people are reporting, I wouldn’t be surprised if the drop rate had been bumped up a bit higher than 10%, perhaps to compensate for earlier frustration. It would be hard to prove this with this data, though, as I suspect that many people stopped grinding immediately after finding one epic key.