It has not directly referred to one way or the other in the series. However, in The Sarah Jane Adventures
episode "Death of the Doctor," The Eleventh Doctor tells Clyde he has 507 regenerations. Some people (myself included) choose to believe he was joking (and the scene itself merits this belief) while others firmly believe it as truth. Of course, this is the same episode where Sarah Jane suggests Ian and Barbara are ageless and immortal and many people don't believe that either.
That all being said, I want the producers to stick to the regeneration limit because I want to see how this affects The Doctor. I want to see The Doctor to completely and utterly face his mortality and not like how The Tenth Doctor did prior to his regeneration. I want to see The Thirteenth Doctor defined by his mortality and having to face his inevitable fate. I want to see how that shapes his adventures for better or for worse. And then when the time comes, have The Doctor actually die and end the series. For good. I know very few people will agree with this but I think that would be an incredible way to end an incredible series.