Well... the Regeneration Limit was mentioned in Classic Who and a few sparse moments in New Who and then recanted by Neil Gaiman and the rest of the writers (Davies/Moffat). The Doctor never said anything about his max number. Simply that there was a limit. He did however sometimes mention what regeneration he was on. But, as we saw with Time Lord Victorious, with the Doctor being 'the last of the Time Lords' there aren't any rules anymore (or at the very least nobody to enforce the rules). The Doctor simply chooses to follow the rules laid down by Rassilon and the other Time Lords. And he imposed his own rules on himself for the safety of those around him. Paraphrased: "A good person doesn't need rules. You will soon learn why I have so many."
Along with that, the Master broke the so-called regeneration limit before the Doctor was close to it. Therefore it isn't necessarily a biological limit but more of a rule. Some people even say that John Simm was the Master's 15th form (but not necessarily regeneration number).
And then there's the fact of when you start counting. After the first regeneration or when you're born.
In regards to the Day of the Doctor, there still isn't much of an explanation for where the Master and Rassilon even are, or were, on the last day of the Last Great Time War. They were both there. The Master fled from the battle. This has been established many times in the show. Even if we account for wibbly wobbly and the star, there should have been something dealing with both of them. But I'm biased, probably.
As for Eccleston, he turned down the role with an explanation of it interrupting filming for Thor. But that couldn't really be true because Thor ended filming as the 50th was starting. There has been some rumors about why Chris only had one season which includes the fact that he didn't get along with the crew and Davies. And vice versa. And with that still hanging around, he declined to be in the 50th because the people he didn't get along with were still involved. It doesn't mean he didn't care about the show itself. Simply that the experience would have been tainted.
“I thought to remain, which would have made me a lot of money and given me huge visibility, the price I would have had to pay was to eat a lot of shit. I’m not being funny about that. I didn’t want to do that and it comes to the art of it, in a way. I feel that if you run your career and– we are vulnerable as actors and we are constantly humiliating ourselves auditioning. But if you allow that to go on, on a grand scale you will lose whatever it is about you and it will be present in your work." -Chris Eccleston
