Its a question we have all wanted to know the answer too and we might just get one this series.It was thought to be 12 lives but we dont actually know for sure.
Speaking to Sci fi site Newsarama Moffats says:
“It’s been addressed in a very, very cheeky way by an old friend of mine and I’m not going to tell you any more about that, wait and see.”
Also Matt Smith had this to say about the series Arc:
Smith told the crowd it’s seen in the very first episode. “Albeit, not in the most conventional form. You’re going to have to think about it and find out what it is, but it’s in there,” he said. “And that’s the one, that’s what takes us through the whole series and my god, it’s bad.”